Story highlights U.S. State Department warns Americans against non-essential travel to Ukraine

"There will be costs for any military intervention in Ukraine," President Barack Obama says

"We are strong enough to defend ourselves," Ukrainian ambassador says

Russian President Vladimir Putin to EU leaders: Ukraine must avoid an escalation of violence

Tension dramatically mounted in Ukraine's Crimea region Friday as its ambassador to the United Nations warned Russia against any further violation of its territorial borders, a warning that came as the United States urged Russia to pull back from the region or face possible consequences.

"We are now deeply concerned by reports of military movements taken by the Russian Federation inside Ukraine," U.S. President Barack Obama said in televised comments from the White House.

"...It would be a clear violation of Russia's commitment to respect the independence and sovereignty and borders of Ukraine and of international laws."

Obama said any violation of Ukraine's sovereignty and territorial integrity would be "deeply destabilizing, and he warned "the United States will stand with the international community in affirming that there will be costs for any military intervention in Ukraine."

The remarks were the latest in a series of fast-moving developments that saw Ukrainian officials grappling with rising secessionist passions in the Russian-majority region, where the airspace has been closed and communications have been disrupted.

Photos: Photos: Crisis in Ukraine Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Ukrainian tanks are transported from their base in Perevalne, Crimea, on Wednesday, March 26. After Russian troops seized most of Ukraine's bases in Crimea, interim Ukrainian President Oleksandr Turchynov ordered the withdrawal of armed forces from the peninsula, citing Russian threats to the lives of military staff and their families. Hide Caption 1 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Ukrainian marines wave as they leave a base in Feodosia, Crimea, on Tuesday, March 25. Hide Caption 2 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Russian sailors stand on the deck of the corvette ship Suzdalets in the bay of Sevastopol, Crimea, on March 25. Hide Caption 3 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Pro-Russian militia members remove a local resident as Russian troops assault the Belbek air base, outside Sevastopol, on Saturday, March 22. Following its annexation of Crimea, Russian forces have consolidated their control of the region. Hide Caption 4 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Soldiers in unmarked uniforms sit atop an armored personnel carrier at the gate of the Belbek air base on March 22. Hide Caption 5 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A Russian sailor holds the Russian Navy's St. Andrew's flag while standing on the bow of the surrendered Ukrainian submarine Zaporozhye on March 22 in Sevastopol. Hide Caption 6 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Russian President Vladimir Putin signs the final decree completing the annexation of Crimea on Friday, March 21, as Upper House Speaker Valentina Matviyenko, left, and State Duma Speaker Sergei Naryshkin watch. Hide Caption 7 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A Ukrainian serviceman leaves a Ukrainian military unit that Russian soldiers took control of in Perevalne on March 21. Hide Caption 8 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Ukrainian border guards run during training at a military camp in Alekseyevka, Ukraine, on March 21. Hide Caption 9 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Russian soldiers patrol the area surrounding a Ukrainian military unit in Perevalne on Thursday, March 20. Hide Caption 10 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Pro-Russian protesters remove the gate to the Ukrainian navy headquarters as Russian troops stand guard in Sevastopol on Wednesday, March 19. Hide Caption 11 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Pro-Russian forces walk inside the Ukrainian navy headquarters in Sevastopol on March 19. Hide Caption 12 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A member of pro-Russian forces takes down a Ukrainian flag at the Ukrainian navy headquarters in Sevastopol on March 19. Hide Caption 13 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Alexander Vitko, chief of the Russian Black Sea Fleet, leaves the Ukrainian navy headquarters in Sevastopol after pro-Russian forces took it over on March 19. Hide Caption 14 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A Russian flag waves as workers install a new sign on a parliament building in Simferopol, Crimea's capital, on March 19. Hide Caption 15 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Russian military personnel surround a Ukrainian military base in Perevalne on March 19. Hide Caption 16 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Nameplates on the front of the Crimean parliament building get removed Tuesday, March 18, in Simferopol. Hide Caption 17 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – From left, Crimean Prime Minister Sergey Aksyonov; Vladimir Konstantinov, speaker of the Crimean parliament; Russian President Vladimir Putin; and Alexei Chaly, the new de facto mayor of Sevastopol, join hands in Moscow on March 18 after signing a treaty to make Crimea part of Russia. Hide Caption 18 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Demonstrators hold a Crimean flag at Lenin Square in Simferopol on March 18. Hide Caption 19 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Ukrainian soldiers stand guard at a checkpoint near Strilkove, Ukraine, close to Crimea on Monday, March 17. Hide Caption 20 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Former boxer and Ukrainian politician Vitali Klitschko addresses reporters in Kiev, Ukraine, on March 17. Hide Caption 21 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Ukrainian troops stand guard in front of the Ukrainian Parliament building in Kiev on March 17. Hide Caption 22 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A Ukrainian man applies for the National Guard at a mobile recruitment center in Kiev on March 17. Hide Caption 23 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Civilians walk past riot police in Simferopol on March 17. Hide Caption 24 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A Ukrainian soldier stands on top of an armored vehicle at a military camp near the village of Michurino, Ukraine, on March 17. Hide Caption 25 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Policemen stand guard outside the regional state administration building in Donetsk, Ukraine, during a rally by pro-Russia activists March 17. Hide Caption 26 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Armed soldiers stand guard outside a Ukrainian military base in Perevalne on March 17. Hide Caption 27 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A man holds a Crimean flag as he stands in front of the Crimean parliament building in Simferopol on March 17. Hide Caption 28 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Crimeans holding Russian flags celebrate in front of the parliament building in Simferopol on Sunday, March 16. Hide Caption 29 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A Ukrainian police officer tries to shield himself from a road block thrown by pro-Russia supporters in Kharkiv, Ukraine, on March 16. Hide Caption 30 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Pro-Russia demonstrators storm the prosecutor general's office during a rally in Donetsk on March 16. Hide Caption 31 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A child casts her mother's ballot March 16 while holding a Russian flag at a polling station in Simferopol. Hide Caption 32 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A pro-Russian soldier, with the Russian flag behind him, mans a machine gun outside an Ukrainian military base in Perevalne on Saturday, March 15. Hide Caption 33 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A pro-Russian soldier guards the perimeter outside an Ukrainian military base in Perevalne on March 15. Hide Caption 34 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Russian flags wave in front of a monument dedicated to Soviet Union founder Vladimir Lenin during a pro-Russia rally in Simferopol's Lenin Square on March 15. Hide Caption 35 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Evgenyi Batyukhov cries March 14 at the site where pro-Russian and pro-Ukrainian activists clashed the night before in Donetsk, Ukraine. Hide Caption 36 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A line of policemen stand in front of a bus of pro-Ukrainian activists as pro-Russian supporters confront them during a rally in Donetsk on Thursday, March 13. Hide Caption 37 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Pro-Russian supporters clash with pro-Ukrainian activists in Donetsk on March 13. Hide Caption 38 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Recent Academy Award winner Jared Leto walks through Independence Square in Kiev on March 13. During his Oscars acceptance speech in early March, the actor spoke to protesters in Ukraine and Venezuela saying, "We're thinking of you tonight." Hide Caption 39 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A member of a pro-Russian self-defense unit checks a polling station near Simferopol on March 13. Hide Caption 40 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A Ukrainian soldier looks out of the window of a regional military building with a poster reading "Ukraine's armed forces wait for you!" in Donetsk on March 13. Hide Caption 41 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – An armed Cossack stands guard at a checkpoint on the road from Simferopol to Sevastopol on March 13. Hide Caption 42 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A pro-Russian soldier speaks to a truck driver outside the Ukrainian infantry base in Perevalne on Wednesday, March 12. Hide Caption 43 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Cossacks stand guard at Crimea's regional parliament building in Simferopol on March 12. Hide Caption 44 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Soldiers are seen aboard the Ukrainian ship Slavutych in the harbor of Sevastopol on Tuesday, March 11. Hide Caption 45 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Ukrainian naval officers board a boat in front of the Russian minesweeper Turbinist in Sevastopol's harbor on March 11. Hide Caption 46 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – People shout slogans during a pro-Russia rally in Donetsk on Sunday, March 9. Hide Caption 47 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Ukrainian police detain a demonstrator during a pro-Russian rally in Donetsk on March 9. Hide Caption 48 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Pro-Russia protesters remove a Ukrainian flag from a flagpole taken from a government building in Donetsk on March 9. Hide Caption 49 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Cossacks and other pro-Russian forces stand guard outside a government building in Simferopol on Saturday, March 8. Hide Caption 50 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Ukrainian soldiers load armored personnel carriers into boxcars in the western Ukrainian city of Lviv on March 8. Hide Caption 51 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Armed men believed to be Russian military march in a village outside Simferopol on Friday, March 7. Hide Caption 52 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Pro-Russia protesters demonstrate outside the Belbek air base, near Sevastopol, on Thursday, March 6. Hide Caption 53 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A Ukrainian navy officer looks at the scuttled, decommissioned Russian vessel Ochakov from the Black Sea shore outside the town of Myrnyi, Ukraine, on March 6. Russian naval personnel scuttled the ship, blockading access for five Ukrainian naval vessels. Hide Caption 54 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A member of the Russian military patrols around Perevalne on March 6. Hide Caption 55 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Servicemen guard a checkpoint at a Ukrainian navy base in Perevalne on March 6. Hide Caption 56 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Ukrainian troops guard the Belbek air base on March 6. Hide Caption 57 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A woman walks past barricades March 6 that were set up by anti-government protesters in Kiev's Independence Square. Hide Caption 58 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A sailor guards the Ukrainian Navy ship Slavutych in the Bay of Sevastopol on Wednesday, March 5. Hide Caption 59 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – People wait in line for food distribution in Independence Square on March 5. Hide Caption 60 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Ukrainian sailors carry meat to their vessel in the Sevastopol harbor on March 5. Hide Caption 61 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Riot police stand at the entrance of a regional administrative building during a rally in Donetsk on March 5. Hide Caption 62 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A Ukrainian police officer gives instructions to members of the media in front of the business class lounge of the Simferopol airport on March 5. Hide Caption 63 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Pro-Russia demonstrators wave a Russian flag after storming a regional administrative building in Donetsk on March 5. Hide Caption 64 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Demonstrators break a police barrier as they storm a regional administrative building in Donetsk on March 5. Hide Caption 65 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Ukrainian military recruits line up to receive instructions in Kiev's Independence Square on Tuesday, March 4. Hide Caption 66 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – People stand on the Ukrainian Navy ship Slavutych while it's at harbor in Sevastopol on March 4. Mattresses were placed over the side of the ship to hinder any attempted assault. Hide Caption 67 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Ukrainian troops watch as a Russian navy ship blocks the entrance of the Ukrainian navy base in Sevastopol on March 4. Hide Caption 68 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A woman photographs pro-Russian soldiers guarding Ukraine's infantry base in Perevalne on March 4. Hide Caption 69 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – U.S. Secretary of State John Kerry, wearing a blue scarf, visits a shrine March 4 for the people who were killed in February during anti-government protests in Kiev. Hide Caption 70 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Yuli Mamchun, the commander of the Ukrainian military garrison at the Belbek air base near Sevastopol, salutes on March 4. Hide Caption 71 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Russian soldiers stand guard at the Belbek air base on March 4. Hide Caption 72 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Ukrainian military members march at the Belbek air base on March 4. Hide Caption 73 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Russian soldiers fire warning shots to keep back Ukrainian military members at the Belbek air base on March 4. Hide Caption 74 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A Ukrainian airman puts the Ukrainian national flag over the gate of the Belbek air base as they guard what's left under their control on March 4. Hide Caption 75 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Russian soldiers aim a grenade launcher and machine gun as they guard positions at the Belbek air base on March 4. Hide Caption 76 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Ukrainian seamen stand guard on the Ukrainian navy ship Slavutych in the Sevastopol harbor on Monday, March 3. Hide Caption 77 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Oleg, a Ukrainian soldier, kisses his girlfriend, Svetlana, through the gates of the Belbek base entrance on March 3. Tensions are high at the base, where Ukrainian soldiers were standing guard inside the building while alleged Russian gunmen were standing guard outside the gates. Hide Caption 78 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Wives of Ukrainian soldiers walk past Russian soldiers to visit their husbands guarding a military base in Perevalne on March 3. Hide Caption 79 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A Russian soldier guards an area outside Ukraine's military base in the village of Perevalne on March 3. Hide Caption 80 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A sailor looks out a window near the entrance to the Ukrainian navy headquarters in Sevastopol on March 3. Hide Caption 81 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Armed men in military uniform walk outside a Ukrainian military unit near Simferopol on Sunday, March 2. Hundreds of armed men in trucks and armored vehicles surrounded the Ukrainian base Sunday in Crimea, blocking its soldiers from leaving. Hide Caption 82 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Soldiers walk outside a Ukrainian military base in Perevalne as a local resident waves a Russian flag March 2. Hide Caption 83 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Demonstrators shout during a rally in Kiev's Independence Square on March 2. Hide Caption 84 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Ukrainian soldiers, left, and unidentified gunmen, right, stand at the gate of an infantry base in Perevalne on March 2. Hide Caption 85 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Ukrainian soldiers guard a gate of an infantry base in Perevalne on March 2. Hide Caption 86 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A woman cries during a rally in Independence Square on March 2. Hide Caption 87 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Protesters hold flags of the United States, Germany and Italy during a rally in Independence Square on March 2. Hide Caption 88 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – People attend a morning prayer service at Independence Square on March 2. Hide Caption 89 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A soldier and a truck driver unload bread outside the Ukranian navy headquarters in Sevastopol on March 2. Hide Caption 90 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Heavily armed troops, displaying no identifying insignia and who were mingling with local pro-Russian militants, stand guard outside a local government building in Simferopol on March 2. Hide Caption 91 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A woman waits in front of unidentified men in military fatigues who were blocking a base of the Ukrainian frontier guard unit in Balaklava, Ukraine, on Saturday, March 1. Hide Caption 92 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – U.S. President Barack Obama, in the Oval Office of the White House, talks on the phone March 1 with Russian President Vladimir Putin. Hide Caption 93 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Troops stand guard in Balaklava on March 1. Hide Caption 94 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Heavily armed soldiers displaying no identifying insignia maintain watch in Simferopol on March 1. Hide Caption 95 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – People gather around the coffin of a man who was killed during clashes with riot police in Independence Square. Hide Caption 96 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Pro-Russian activists hold Russian flags during a rally in the center of Donetsk on March 1. Hide Caption 97 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Pro-Russian activists clash with Maidan supporters as they storm the regional government building in Kharkiv, Ukraine, on March 1. Hide Caption 98 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A protester stands at a memorial March 1 for the people killed in clashes at Independence Square. Hide Caption 99 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Armed men patrol outside the Simferopol International Airport on Friday, February 28. Hide Caption 100 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – An image provided to CNN by a local resident shows Russian tanks on the move in Sevastopol. Hide Caption 101 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Russian troops block a road February 28 toward the military airport in Sevastopol. The Russian Black Sea Fleet is based at the port city. Hide Caption 102 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Armed men stand guard in front of a building near the Simferopol airport on February 28. Hide Caption 103 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – An armed man wearing no identifying insignia patrols outside Simferopol International Airport on February 28. Hide Caption 104 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Police stand guard outside the Crimea regional parliament building Thursday, February 27, in Simferopol. Armed men seized the regional government administration building and parliament in Crimea. Hide Caption 105 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Police intervene as Russian supporters gather in front of the parliament building in Simferopol on February 27. Hide Caption 106 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A man adds fuel to a fire at a barricade in Independence Square on February 27. Dozens of people were killed during clashes between security forces and protesters. Hide Caption 107 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Pro-Russia demonstrators wave Russian and Crimean flags in front of a local government building in Simferopol on February 27. Hide Caption 108 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Barricades in front of a government building in Simferopol on February 27 hold a banner that reads: "Crimea Russia." There's a broad divide between those who support the pro-Western developments in Kiev and those who back Russia's continued influence in Crimea and across Ukraine. Hide Caption 109 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Protesters stand in front of a government building in Simferopol on February 27. Tensions have simmered in the Crimea region since the ouster of Ukrainian President Viktor Yanukovych. Hide Caption 110 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Protesters in support of the president's ouster rally in Independence Square, which has been the center of opposition, on Wednesday, February 26. Hide Caption 111 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Security forces stand guard during clashes between opposing sides in front of Crimea's parliament building in Simferopol on February 26. Hide Caption 112 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Pro-Russian demonstrators, right, clash with anti-Russian protesters in front of a government building in Simferopol on February 26. Hide Caption 113 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A police officer gets pulled into a crowd of Crimean Tatars in Simferopol on February 26. The Tatars, an ethnic minority group deported during the Stalin era, rallied in support of Ukraine's interim government. Hide Caption 114 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A man places flowers at a barricade near Independence Square on February 26. Hide Caption 115 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – On February 26 in Kiev, a woman holds a photograph of a protester killed during the height of tensions. Hide Caption 116 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Police guard a government building in Donetsk on February 26. Hide Caption 117 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Protesters remove a fence that surrounds Ukraine's parliament in Kiev on February 26. Hide Caption 118 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – People sing the Ukrainian national anthem at Independence Square on Monday, February 24. Hide Caption 119 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Gas masks used by protesters sit next to a barricade in Independence Square on February 24. Hide Caption 120 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A woman cries February 24 near a memorial for the people killed in Kiev. Hide Caption 121 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – People wave a large Ukrainian flag in Independence Square on Sunday, February 23. Hide Caption 122 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Two pro-government supporters are made to pray February 23 in front of a shrine to dead anti-government protesters. Hide Caption 123 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A man and his daughter lay flowers at a memorial for protesters killed in Independence Square. Hide Caption 124 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Former Prime Minister Yulia Tymoshenko speaks at Independence Square on Saturday, February 22, hours after being released from prison. Tymoshenko, considered a hero of a 2004 revolution against Yanukovych, was released after 2½ years behind bars. Hide Caption 125 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Tymoshenko is greeted by supporters shortly after being freed from prison in Kharkiv on February 22. Hide Caption 126 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – A protester guards the entrance to Yanukovych's abandoned residence outside Kiev on February 22. Hide Caption 127 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Anti-government protesters guard the streets next to the presidential offices in Kiev on February 22. Hide Caption 128 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Anti-government protesters drive a military vehicle in Independence Square on February 22. Many protesters said they wouldn't leave the square until Yanukovych resigned. Hide Caption 129 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Ukrainian lawmakers argue during a session of Parliament on Friday, February 21. Hide Caption 130 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Men in Kiev carry a casket containing the body of a protester killed in clashes with police. Hide Caption 131 of 132 Photos: Photos: Crisis in Ukraine Crisis in Ukraine – Protesters cheer after news of an agreement between the opposing sides in Kiev on February 21. Hide Caption 132 of 132

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Ukraine accused Russian Black Sea forces of trying to seize two airports in Crimea but said Ukrainian security forces prevented them from taking control.

Ukraine Interior Minister Arsen Avakov earlier characterized the presence at the airport of unidentified armed men, who wore uniforms without insignia, as an "armed invasion."

The crisis echoed throughout the world, with the U.N. Security Council president holding a private meeting about the crisis enveloping Ukraine and world leaders calling armed groups not to attempt to challenge Ukrainian sovereignty.

'This group is making a serious mistake'

At a press conference outside the U.N. Security Council, Ukraine's ambassador to the U.N., Yuriy Sergeyev said the country was prepared to defend itself and urged the U.N.'s moral and political support for the Kiev government, particularly in Crimea.

Since last week's ouster of President Viktor Yanukovych, Ukraine has faced a deepening schism, with those in the west generally supporting the interim government and its European Union tilt, while many in the east preferring a Ukraine where Russia casts a long shadow.

Nowhere is that feeling more intense than in Crimea, the last big bastion of opposition to the new political leadership. And Ukraine suspects Russia of fomenting tension in the autonomous region that might escalate into a bid for separation by its Russian majority.

"We still have a chance to stop the negative developments and separatism," Sergeyev said.

Sergeyev accused Russia of violating its military agreement by blocking Ukrainian security forces, including its border guards and police, in the region.

"This group is making a serious mistake challenging our territorial integrity," he said.

But Russia's ambassador to the United Nations, Vitaliy Churkin, compared the reports of Russian troops taking charge of positions on the ground to rumors that "are always not true."

"We are acting within the framework of our agreement," he said.

Even so, U.S. military commanders and intelligence agencies were scrambling Friday to determine what was needed to get a better picture of Russian movements.

That included an assessment of intelligence gathering, surveillance and reconnaissance needs, a senior U.S. official told CNN.

The U.S. State Department warned Americans to defer all non-essential travel to Ukraine, particularly the Crimea region, "due to the potential for instability following the departure of former President Yanukovych and the establishment of a new government."

Meanwhile, Obama is considering not attending the G8 Summit in Sochi, Russia, in June, if Russian troops remain in the Ukraine, a senior administration official familiar with the discussions told CNN.

Both officials spoke on condition of anonymity.

Kerry talks to Russian foreign minister

The Russian Foreign Ministry said maneuvers of armored vehicles from the Russian Black Sea Fleet in Crimea were needed for security and were in line with bilateral agreements.

U.S. Secretary of State John Kerry said he spoke with Russian Foreign Minister Sergey Lavrov on Friday morning about the airport and military activities, and Lavrov told Kerry that the Russians "are not engaging in any violation of the sovereignty" of Ukraine. Russia has a military base agreement with the country.

Lavrov told him the military exercises were prescheduled and unrelated to the events in Ukraine, Kerry said.

"I nevertheless made it clear that that could be misinterpreted at the moment,'' Kerry said, "and there are enough tensions that it is important for everybody to be extremely careful not to inflame the situation and send the wrong messages."

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Kerry said Lavrov had reaffirmed to him a commitment that Russia would "respect the territorial integrity of Ukraine."

"We would overwhelmingly stress today that we urge all parties -- all parties; that includes the new interim technical government, rightists, oppositionists and others, anybody in the street who is armed -- we urge all parties to avoid any steps that could be misinterpreted or lead to miscalculation or do anything other than to work to bring that peace and stability and peaceful transition within the governing process within Ukraine," Kerry said.

Russian response

In a telephone call with European leaders, Russian President Vladimir Putin stressed the importance of avoiding a further escalation of violence in Ukraine, the Kremlin said in a prepared statement Friday.

Putin also called for a normalization of the situation, speaking with German Chancellor Angela Merkel, British Prime Minister David Cameron and European Council President Herman Van Rompuy, according to the Kremlin.

Crimea was handed to Ukraine by the Soviet Union in 1954. Just over half its population is ethnic Russian, while about a quarter are Ukrainians and a little more than 10% are Crimean Tatars, a predominantly Muslim group oppressed under former Soviet leader Josef Stalin.

Meanwhile, Russian lawmakers introduced two bills Friday to simplify annexing new territories into the Russian Federation and simplify access to Russian citizenship for Ukrainians, the state news agency Itar Tass said.

One bill also stipulates that the accession of a part of a foreign state to Russia should be taken through a referendum, according to Russian state news agency RIA Novosti.

Ukraine's President in Russia

Making his first public appearance since his ouster Saturday, Ukrainian President Viktor Yanukovych said the newly appointed interim government was not legitimate and did not represent the majority of Ukraine's 45 million citizens.

"I intend to continue the fight for the future of Ukraine against those who, with fear and with terror, are attempting to replace the power," Yanukovych said in Russian, not Ukrainian.

"Nobody has overthrown me. I was compelled to leave Ukraine due to a direct threat to my life and my nearest and dearest."

In his hourlong news conference, Yanukovych accused the interim authorities in Ukraine of propagating violence. He spoke against a backdrop of Ukraine's blue-and-yellow flags before reporters in the Russian city of Rostov-on-Don about 700 miles south of Moscow.

"I never gave any orders to shoot," he said, adding that he sought peace and that the security forces took up arms only when their lives were at risk.

Yanukovych is wanted in Ukraine on charges connected to the deaths of demonstrators, who were protesting his decision to scrap a European Union trade deal in favor of one with Russia.

Armed men at airports

Back in Kiev, Andrii Parubii, chief of national security and defense, said Ukrainian military and police forces had stopped Russian military forces from seizing two airports in the Crimean region.

The Russian military is on the outside of both airports, Parubii said in a televised news conference from the Ukrainian parliament.

Weapons were not used during the operation, according to Avakov, the interior minister.

Russian armored vehicles were moving toward Simferopol, the regional capital, on Friday, the Ukrainian news outlet TSN reported.

Men in military uniforms had been seen patrolling the airport in Simferopol, as well as a military and civilian airbase in nearby Sevastopol since early Friday.

Avakov said the armed men at the Sevastopol air base were troops from Russia's Black Sea Fleet, stationed in the port city. They were in camouflage uniforms without military insignia, he said.

The presence of the armed men has not affected the Simferopol airport, civil aviation authorities said.

"We are checking to make sure that no radicals come to Crimea from Kiev, from the Ukraine," said one man outside the airport, who didn't give his name. "We don't want radicals, we don't want fascism, we don't want problems."

Other men outside the airport, dressed in black rather than military fatigues, said they belonged to the pro-Russia Unity Party and had come on the orders of the new Crimean administration -- voted in Thursday after armed men seized regional government buildings.

Concerned about the latest developments, Ukraine's parliament passed a resolution Friday that demanded Russia halt any activity that can be interpreted as an attack on its sovereignty.

Moscow alarmed some observers by announcing the surprise military exercises Wednesday in its western and central areas, near the Ukraine border.

Meanwhile, Ukraine's largest telecom firm was unable to provide data and voice connectivity between Crimea and the rest of Ukraine because unknown people had seized telecommunications nodes and destroyed cables, it said Friday. There is almost no phone connectivity or Internet service across Crimea, said Ukrtelecom, which is the only landline provider.