Story highlights Ukraine says Russian troops on its soil in region that controls natural gas, water to Crimea

Russians reportedly say they are there to prevent terrorist attacks on oil assets

Russia vetoes resolution declaring Crimea referendum invalid

Thousands of protesters rally in Ukraine and Moscow ahead of Sunday referendum

Russia asserted its diplomatic and military muscle Saturday as tensions remained high ahead of a vote on secession in Crimea.

About 60 Russian troops in six helicopters and three armored vehicles reportedly crossed into Ukraine's Kherson region and were in the town of Strilkove, on a strip of land just northeast of Crimea.

The State Border Guard Service of Ukraine said the Russian troops were on the ground and the Ukrainian guards had taken defensive positions.

The region is key to neighboring Crimea, because it gets electricity, freshwater and natural gas from there. The Russians said they were in Kherson to prevent a possible terrorist attack on oil assets in the area, according to the Ukrainian border guards.

Photos: Photos: Crisis in Crimea and Ukraine Photos: Photos: Crisis in Crimea and Ukraine Crisis in Crimea and Ukraine, captured by CNN teams – SLOVYANSK, UKRAINE: "Welcome to Balaclavistan! This happy chap wouldn't say if he's Crimean, Russian or Ukrainian." - CNN's Christian Streib. Follow Christian on Instagram at instagram.com/christianstreibcnn Hide Caption 1 of 44 Photos: Photos: Crisis in Crimea and Ukraine Crisis in Crimea and Ukraine, captured by CNN teams – EASTERN UKRAINE: "Ukrainian Police checkpoint north of Donetsk. A rare sight these days in Eastern Ukraine." - CNN's Christian Streib, April 15. Follow Christian on Instagram at instagram.com/christianstreibcnn Hide Caption 2 of 44 Photos: Photos: Crisis in Crimea and Ukraine Crisis in Crimea and Ukraine, captured by CNN teams – NEAR KIEV, UKRAINE: "Fresh recruits to Ukraine's National Guard Reserves (March 20). All of them are former anti-government militiamen from Kyiv's Maydan. They've all signed up within the last week as part of a mass recruitment program announced by the interim Ukrainian government." - CNN's Ivan Watson. WATCH THE INSTAGRAM VIDEO from Ivan of a curious "hand grenade" simulation exercise performed by new Ukrainian National Guard reserve recruits. Follow Ivan on Instagram at instagram.com/ivancnn Hide Caption 3 of 44 Photos: Photos: Crisis in Crimea and Ukraine Crisis in Crimea and Ukraine, captured by CNN teams – SIMFEROPOL, CRIMEA: "Early morning departure (March 19) at Simferopol airport where the Russian flag is already firmly in place on the tarmac." - CNN's Dominique Van Heerden. Follow Dominique on Instagram at instagram.com/dominique_vh Hide Caption 4 of 44 Photos: Photos: Crisis in Crimea and Ukraine Crisis in Crimea and Ukraine, captured by CNN teams – SIMFEROPOL, UKRAINE: "Celebrations already well underway here in Lenin Square in Simferopol (March 16)." - CNN's Dominique Van Heerden. Follow Dominique on Instagram at instagram.com/dominique_vh Hide Caption 5 of 44 Photos: Photos: Crisis in Crimea and Ukraine Crisis in Crimea and Ukraine, captured by CNN teams – KIEV, UKRAINE: "The mood was grim in Kyiv's Maydan (March 16), as a mustachioed Ukrainian Cossack beat a kettle drum during the separatist referendum in Crimea." - CNN's Ivan Watson. WATCH THE INSTAGRAM VIDEO Follow Ivan on Instagram at instagram.com/ivancnn Hide Caption 6 of 44 Photos: Photos: Crisis in Crimea and Ukraine Crisis in Crimea and Ukraine, captured by CNN teams – SIMFEROPOL, UKRAINE: "Voting has started in Crimea (March 16). Steady stream of voters at this polling station in the center of Simferopol." - CNN's Dominique Van Heerden. Follow Dominique on Instagram at instagram.com/dominique_vh Hide Caption 7 of 44 Photos: Photos: Crisis in Crimea and Ukraine Crisis in Crimea and Ukraine, captured by CNN teams – DONETSK, UKRAINE: Demonstrators wave revolutionary flags at a major pro-Russia rally in Lenin Square in central Donetsk (March 15). Some are calling for independence from Ukraine. Others are calling for union with Russia. Photo by CNN's Kellie Morgan. Hide Caption 8 of 44 Photos: Photos: Crisis in Crimea and Ukraine Crisis in Crimea and Ukraine, captured by CNN teams – SIMFEROPOL, UKRAINE: "Pro-Soviet flags being made and handed out in Simferopol on March 14." - CNN's Michael Holmes. Follow Michael on Instagram at instagram.com/holmescnn Hide Caption 9 of 44 Photos: Photos: Crisis in Crimea and Ukraine Crisis in Crimea and Ukraine, captured by CNN teams – SIMFEROPOL, UKRAINE: "You really don't have to wander very far in Simferopol (March 14) to find posters like this one around parliament and Lenin Square." - CNN's Dominique Van Heerden. Follow Dominique on Instagram at instagram.com/dominique_vh Hide Caption 10 of 44 Photos: Photos: Crisis in Crimea and Ukraine Crisis in Ukraine, captured by CNN Teams on assignment. – KIEV, UKRAINE: "Protesters outside parliament in Kiev on March 12 call for the release of 34 political prisoners arrested during demonstrations." - CNN's Dominique Van Heereden. Follow Dominique on Instagram at instagram.com/dominique_vh Hide Caption 11 of 44 Photos: Photos: Crisis in Crimea and Ukraine Crisis in Ukraine, captured by CNN Teams on assignment. – DONETSK, UKRAINE: "Dominating the main square named after him: Vladimir Ilyich Ulyanov, also known as Lenin" - CNN's Christian Streib. Follow Christian on Instagram at instagram.com/christianstreibcnn Hide Caption 12 of 44 Photos: Photos: Crisis in Crimea and Ukraine Crisis in Ukraine, captured by CNN Teams on assignment. – KIEV, UKRAINE: CNN cameraman Scott McWhinnie films volunteers peeling vegetables, preparing meals for protesters still living in tents in Maidan, also known as Independence Square, on March 9. Photo by CNN's Michael Holmes. Follow Michael on Instagram at instagram.com/holmescnn Hide Caption 13 of 44 Photos: Photos: Crisis in Crimea and Ukraine Crisis in Ukraine, captured by CNN Teams on assignment. – SEVASTOPOL, UKRAINE: The Ukranian Navy vessel Slavutych remains blocked by Russian Navy boats inside the Port of Sevastopol on March 10, photographed by CNN's Christian Streib. Follow Christian on Instagram at instagram.com/christianstreibcnn Hide Caption 14 of 44 Photos: Photos: Crisis in Crimea and Ukraine Crisis in Ukraine, captured by CNN Teams on assignment. – SEVASTOPOL, UKRAINE: "CNN's Matthew Chance and team talk to the first officer of the Ukrainian Intelligence Navy ship Slavutych inside the Port of Sevastopol on March 10." - CNN's Christian Streib. Follow Christian on Instagram at instagram.com/christianstreibcnn Hide Caption 15 of 44 Photos: Photos: Crisis in Crimea and Ukraine Crisis in Ukraine, captured by CNN Teams on assignment. – KIEV, UKRAINE: People stream through Independence Square, laying flowers, and lighting candles. like the ones pictured here on March 9. Photo by CNN's Dominique Van Heerden. Follow Dominique on Instagram at instagram.com/dominique_vh Hide Caption 16 of 44 Photos: Photos: Crisis in Crimea and Ukraine Crisis in Ukraine, captured by CNN Teams on assignment. – KIEV, UKRAINE: An aerial photo of Independence Square, or Maidan, in Kiev on March 8. Many remain camped out in Independence Square, while others stream through the area to pay tribute to those who were killed. Photo by CNN's Tony Umrani. Hide Caption 17 of 44 Photos: Photos: Crisis in Crimea and Ukraine Crisis in Ukraine, captured by CNN Teams on assignment. – KIEV, UKRAINE: A giant EU flag is hung outside the foreign ministry building in Kiev on March 7. Photo by CNN's Dominique Van Heerden. Follow Dominique on Instagram at instagram.com/dominique_vh Hide Caption 18 of 44 Photos: Photos: Crisis in Crimea and Ukraine Ukraine Crisis captured by CNN Teams – SOUTHERN UKRAINE: Gunmen block monitors from the Organization for Security and Co-operation in Europe (OSCE) who are trying unsuccessfully to negotiate their way into Crimea past pro-Russian border patrols on March 7. Photo by CNN's Christian Streib. Follow Christian on Instagram at instagram.com/christianstreibcnn Hide Caption 19 of 44 Photos: Photos: Crisis in Crimea and Ukraine Crisis in Ukraine, captured by CNN Teams on assignment. – KIEV, UKRAINE: CNN's Anderson Cooper reports from Kiev's Independence Square on March 6. Photo by CNN's Khalil Abdallah. Hide Caption 20 of 44 Photos: Photos: Crisis in Crimea and Ukraine Ukraine crisis captured by CNN Teams – KIEV, UKRAINE: Portraits of people killed during clashes with riot police are left with candles and flowers at Kiev's Independence Square on March 6. Photo by CNN's Michael Holmes. Follow Michael on Instagram at instagram.com/holmescnn Hide Caption 21 of 44 Photos: Photos: Crisis in Crimea and Ukraine Ukraine Crisis captured by CNN Teams – SEVASTOPOL, UKRAINE: Cameraman Chris Jackson captures the Russian Black Sea fleet in Sevastopol on March 5. Photo by CNN's Ben Wedeman. Follow Ben on Instagram at instagram.com/bcwedeman Hide Caption 22 of 44 Photos: Photos: Crisis in Crimea and Ukraine Ukraine Crisis captured by CNN Teams – KIEV, UKRAINE: "Cutting wood to keep volunteers warm as they continue to guard Independence Square in Kiev." - CNN's Khalil Abdallah on March 5. Follow Khalil on Instagram at instagram.com/madcameraman Hide Caption 23 of 44 Photos: Photos: Crisis in Crimea and Ukraine Ukraine crisis captured by CNN Teams – WASHINGTON, DC: Ukrainian demonstrators rally on March 5 outside the White House against the Russian incursion into Crimea. Photo by CNN's Burke Buckhorn. Follow Burke on Instagram at instagram.com/bbuckhorncnn Hide Caption 24 of 44 Photos: Photos: Crisis in Crimea and Ukraine Ukraine crisis captured by CNN Teams – SEVASTOPOL, UKRAINE: "A massive statue on a hill overlooking Sevastopol, dedicated to the unity of the Soviety army and navy during World War II. After a brutal 250-day siege by German forces, Sevastopol fell in 1942. The past weighs heavily here, with an intense awareness of history, of identity, of place." - CNN's Ben Wedeman. Follow Ben on Instagram at instagram.com/bcwedeman Hide Caption 25 of 44 Photos: Photos: Crisis in Crimea and Ukraine Ukraine crisis captured by CNN Teams – SEVASTOPOL, UKRAINE: "A weary Ukrainian soldier after a five-day standoff at the Belbak air base outside Sevastopol, Crimea, on March 5." - CNN's Ben Wedeman. Follow Ben on Instagram at instagram.com/bcwedeman Hide Caption 26 of 44 Photos: Photos: Crisis in Crimea and Ukraine Ukraine crisis captured by CNN Teams – KIEV, UKRAINE: The Shrine of the Fallen on Institutska Street honors the "Heroes" killed in clashes with police. Photo on March 4 by CNN's Dominique Van Heerden. Follow Dominique on Instagram at instagram.com/dominique_vh Hide Caption 27 of 44 Photos: Photos: Crisis in Crimea and Ukraine Ukraine crisis captured by CNN Teams – KIEV, UKRAINE: CNN Cameraman Christian Streib sets up for live shots by the barricades on the road to Independence Square on March 4. Photo by CNN's Jon Steward. Hide Caption 28 of 44 Photos: Photos: Crisis in Crimea and Ukraine Ukraine crisis captured by CNN Teams – KIEV, UKRAINE: "27-year old Bogdan sits on top of his Russian-made soup kitchen in Indepence -- or Maidan -- Square on March 2. He's been sitting there for the last three months serving delicious Ukrainian grechaniy soup. It's made of buckwheat, lentil, coriander and beef. Amazing taste." - CNN's Christian Streib. Follow Christian on Instagram at instagram.com/christianstreibcnn Hide Caption 29 of 44 Photos: Photos: Crisis in Crimea and Ukraine Ukraine crisis captured by CNN Teams – PEREVALNOYE BASE, UKRAINE: "Outside Ukrainian base in Crimea (March 2), tight-lipped but obviously Russian marines deploy around perimeter as Ukrainian soldiers inside vow not to surrender." - CNN's Ben Wedeman. Follow Ben on Instagram at instagram.com/bcwedeman Hide Caption 30 of 44 Photos: Photos: Crisis in Crimea and Ukraine Ukraine Crisis captured by CNN Teams – KIEV, UKAINE: "Perhaps the best solution to end the crisis in Ukraine." - CNN's Christian Streib on March 2. Follow Ben on Instagram at instagram.com/bcwedeman Hide Caption 31 of 44 Photos: Photos: Crisis in Crimea and Ukraine Ukraine crisis captured by CNN Teams – KIEV, UKRAINE: "APC in central Kiev (March 1). It appeared the morning after Russian forces moved into the Crimea." -- CNN's Ian Lee. Follow Ian on Instagram at instagram.com/ianjameslee Hide Caption 32 of 44 Photos: Photos: Crisis in Crimea and Ukraine Ukraine crisis captured by CNN Teams – KIEV, UKRAINE: A young girl pays tribute to anti-government protesters killed in the clashes with riot police by placing a flower on a makeshift memorial leading to the barricades in central Kiev on February 24. Photo by CNN's Todd Baxter. Hide Caption 33 of 44 Photos: Photos: Crisis in Crimea and Ukraine Crisis in Ukraine, captured by CNN Teams on assignment. – KIEV, UKRAINE: A woman mourns at a makeshift memorial (February 23) in homage to anti-government protesters killed in clashes with riot police in Kiev's Independence Square. Photo by CNN's Todd Baxter. Hide Caption 34 of 44 Photos: Photos: Crisis in Crimea and Ukraine Crisis in Ukraine, captured by CNN Teams on assignment. – KIEV, UKRAINE: Bullet holes in a power pole at a spot where a protester was killed during clashes with riot police, near Independence Square. Photo by CNN's Todd Baxter on February 23. Hide Caption 35 of 44 Photos: Photos: Crisis in Crimea and Ukraine Ukraine crisis captured by CNN Teams – KIEV, UKRAINE: Ukrainians are reflected in a puddle as they gather to mourn the dead in Maidan Square on February 23, after protesters succeeded in forcing President Viktor Yanukovich out of office. Photo by CNN's Christian Streib. Follow Christian on Instagram at instagram.com/christianstreibcnn Hide Caption 36 of 44 Photos: Photos: Crisis in Crimea and Ukraine Ukraine crisis captured by CNN Teams – KIEV, UKRAINE: "Anti-government demonstrator in makeshift riot gear (February 22), a member of several protection units set up by the organizers of the occupation of Maidan Square." - CNN's Christian Streib. Follow Christian on Instagram at instagram.com/christianstreibcnn Hide Caption 37 of 44 Photos: Photos: Crisis in Crimea and Ukraine Ukraine crisis captured by CNN Teams – KIEV, UKRAINE: Defected policemen and anti-government protesters at barricades together in central Kiev on February 21. Photo by CNN's Fred Pleitgen. Follow Fred on Instagram at instagram.com/fpleitgencnn Hide Caption 38 of 44 Photos: Photos: Crisis in Crimea and Ukraine Crisis in Crimea and Ukraine, captured by CNN teams – KIEV, UKRAINE: Anti-government protesters clash with riot police in central Kiev on February 20. Photo by CNN's Todd Baxter. Hide Caption 39 of 44 Photos: Photos: Crisis in Crimea and Ukraine Crisis in Crimea and Ukraine, captured by CNN teams – KIEV, UKRAINE: Riot police face anti-government protesters during clashes in central Kiev on February 20. Photo by CNN's Todd Baxter. Hide Caption 40 of 44 Photos: Photos: Crisis in Crimea and Ukraine Ukraine crisis captured by CNN Teams – KIEV, UKRAINE: After the deaths of 25 people during clashes a day earlier, Ukrainian protesters prepare to stand and fight again on February 19. Photo taken by CNN's Andrew Carey on February 19. Hide Caption 41 of 44 Photos: Photos: Crisis in Crimea and Ukraine Crisis in Crimea and Ukraine, captured by CNN teams – KIEV, UKRAINE: "This 17 year old protestor was taken by police as he was watching the clashes (January 23) while taking iPhone photos. He was stripped naked, beaten, stabbed in the thigh and his arm was broken. He faces eight to fifteen years in jail for taking part in a mass demonstration. The government has labeled those taking part in the violence as 'terrorists'." - CNN's Diana Magnay. Hide Caption 42 of 44 Photos: Photos: Crisis in Crimea and Ukraine Crisis in Crimea and Ukraine, captured by CNN teams – KIEV, UKRAINE: Pro-Europe protesters pour into Independence square on December 14. Opposition supporters have been camping since Nov. 21 in Independence Square - in protest against President Yanukovich's last minute refusal to sign an agreement bringing Ukraine closer to the European Union, in favor of Russia. Photo by CNN's Diana Magnay. Hide Caption 43 of 44 Photos: Photos: Crisis in Crimea and Ukraine Crisis in Crimea and Ukraine, captured by CNN teams – KIEV, UKRAINE: Ukrainian riot police storm barricades set up by pro-European Union protesters in Independence Square on December 11. Ukrainian security forces stormed the square, which protesters have occupied for three weeks. The demonstrators defiantly refused to leave and resisted the police in a tense standoff. Photo by CNN's Diana Magnay. Hide Caption 44 of 44

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Ukraine's foreign ministry termed the move a "military invasion" and called on Russia to withdraw its forces.

Also Saturday, Russia wielded it's veto power as a permanent member of the U.N. Security Council against a U.S. draft resolution that declared Sunday's Crimea referendum invalid.

Thirteen of the 15 Security Council members backed the resolution, while China abstained.

"The reason only one country voted 'no' today is that the world believes that international borders are more than mere suggestions," said U.S. Ambassador Samantha Power after the vote.

Russian Ambassador Vitaly Churkin argued that Crimean citizens had a right to self-determination.

What happens next in Crimea?

Blue white and red Russian flags dominate the streets of the coastal Crimean town of Sevastopol, where concerts on the main square have been celebrating the return to the "motherland" this past week.

"Everybody believes the results are already rigged," said CNN iReporter Maia Mikhaluk from Kiev.

"People are concerned what is going to happen after the referendum," she said. "People are concerned that the Russian army will use force, guns to push (the) Ukrainian army from Crimea."

The referendum will present Crimean residents with the choice of whether to secede from Ukraine and join Russia or effectively become independent.

Ukraine's acting foreign minister, Andrii Deshchytsia, repeated Saturday the interim government's position that the referendum is illegitimate and will not be recognized by Kiev or the international community.

Ukraine is engaged in a "diplomatic war" with Russia, he said, but is looking for a peaceful resolution to the crisis in Crimea. It will not respond to Russian "provocations," he said.

Russia -- which the West and Ukraine insist has the majority ethnic Russian Crimean peninsula under its effective military control -- has come under concerted international pressure to halt its activities there and talk to the interim government in Kiev.

But, so far, it has refused to budge.

Moscow has repeatedly denied direct involvement in Crimea, saying that the well-armed men in uniforms without identifying insignia are not Russian troops.

Protests for both sides

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In the eastern Ukraine city of Donetsk, thousands of pro-Russia demonstrators rallied beneath a towering statue of Soviet revolutionary Vladimir Lenin in the city's main square.

They waved Russian flags and red flags emblazoned with the iconic image of Argentinian Marxist revolutionary Che Guevara.

There was not a single Ukraine flag to be seen, according to a CNN team on the ground.

"They are frightened," said local journalist Denis Kazansky of pro-Ukraine protesters. "They will not come out and demonstrate."

A man was fatally stabbed and at least 10 others were injured in clashes in Donetsk on Thursday. Russia blamed "right-wing radical groups" for the violence, while Ukrainian authorities suggested it was linked to Russian citizens who want to escalate tensions in Ukraine.

Many people living in Ukraine's eastern region, along the Ukraine-Russia border, identify more with Russia than with Ukraine.

Saturday, protesters also took to the streets in the Russian capital, Moscow.

State news agency Itar-Tass, citing Moscow police, said some 15,000 turned out for a rally in support of the referendum in Crimea, while about 3,000 marched in protest against attempts to reunite Crimea with Russia.

The Black Sea peninsula was part of Russia until Soviet leader Nikita Khrushchev gave it to Ukraine in 1954. Ukraine was then part of the Soviet Union. But Russia maintains its Black Sea Fleet in Crimea.

Lavrov: No "common vision"

Despite U.S. warnings of "costs" for Russia if the vote goes ahead, Moscow has refused to budge on the crisis.

Lengthy talks between U.S. Secretary of State John Kerry and his Russian counterpart, Sergey Lavrov, in London on Friday resulted in little more than an acknowledgment of the gap between them.

As Lavrov said, "We don't have a common vision."

Meanwhile, thousands of Russian troops continued a large-scale military drill near the Ukraine border. On the same day, armored trucks toting long-range guns and other military equipment rumbled through northern Crimea. At least one of the vehicles seen by CNN had Russian license plates.

Already, eight Ukrainian military units in Crimea have been taken over, 22 others are blocked, and 49 of 56 border patrol stations are in similar straits, Ukraine's foreign ministry said.

Kiev's new Western-leaning government, which came to power following the ouster of President Viktor Yanukovych, has insisted that Ukraine's territorial integrity, including Crimea, must be respected.

Moscow has insisted it has the right to protect ethnic Russians in Ukraine, who it claims are threatened by radical nationalists and "fascists."

Dispute continues

Estonia's defense minister, Urmas Reinsalu, said Friday that Ukraine is on the verge of a full-scale military conflict unless Russian President Vladimir Putin's hand is forced.

"It is clear that we are at a crossroads," U.N. Secretary-General Ban Ki-moon said. "If positions continue to harden and rhetoric continues to sharpen, there is great risk of a dangerous downward spiral."

Russia so far has refused to engage in direct talks with Ukraine's new leaders, although Lavrov said some communication continues with the interim government in Kiev.

Moscow has also denied any direct involvement in Crimea, saying what's happening in the Black Sea peninsula is an internal matter.

Russia has a major naval base in the Crimean port city of Sevastopol, and thousands of its troops are stationed there.

Sanctions warning

European nations and the United States have announced some targeted punishments against Russia in addition to offers of billions in aid to Ukraine's fledgling government.

Russia has seen steep declines in its stock market and the value of the ruble in recent days. The State Department has cautioned Americans about traveling to Russia given "the possibility of violence or anti-U.S. actions directed against U.S. citizens or U.S. interests."

Kerry warned Friday that the "consequences" will be far more stringent should Putin sign off on the "back-door annexation" of Crimea. Lawmakers in Moscow are due to vote next Friday on accepting Crimea into the Russian Federation should there be a "yes" vote in the referendum.

At the same time, Kerry opened the door for less strident measures should Russia opt not to take in Crimea -- whatever the referendum says -- and instead go along with greater autonomy.