As Ukraine accused Russia of a "military invasion and occupation" on Friday, the United States warned Russia that intervention in Ukraine would be a "grave mistake." Ukraine said Russian troops had taken up positions around a coast guard base and two airports on its strategic Crimean Peninsula. The Kremlin kept silent on the accusations, as the crisis deepened between two of Europe's largest countries.

The UN Security Council met Friday afternoon to discuss the crisis. Ukraine's UN Ambassador Yuriy Sergeyev said he told the UNSC that Russian military helicopters and transport planes are entering his country, and that Russian armed forces seized Ukraine's main airport. He told reporters outside the council that neither major airport in Ukraine is under national control and that the main airport was "captured by Russian armed forces." He said 11 Russian military helicopters had been brought in along with M-24 military transport helicopter. Sergeyev said Russian "groupies" in Crimea are challenging Ukraine's national authority.

Earlier Friday, Interfax news agency quoted a military source as saying that armed men extended their control at a military airport in the Crimea by taking over the runway. "Around 400 people are in the airport of Belbek now. They have occupied runway and all plane movements have been stopped," the news agency quoted the source as saying.

Ukrainian International, Ukraine's biggest airline, said Friday the airspace over the Crimea region had been closed after its main airport was taken over by armed men.

Interfax reported earlier on Friday that no flights from the national capital Kiev were being allowed to Simferopol international airport. But an airport information official said only one flight from Kiev had been delayed, and other flights had been coming and going from Simferopol without any problems.

At the same time, AP journalists in Crimea have spotted a convoy of nine Russian armored personnel carriers and a truck on a road between the port city of Sevastopol and the regional capital, Sinferopol.

The Russian tricolor flags were painted on the vehicles, which were parked on the side of the road near the town of Bakhchisarai, apparently because one of them had mechanical problems.

Russia is supposed to notify Ukraine of any troop movements outside the naval base it maintains in Sevastopol under a lease agreement with Ukraine.

Ukraine's acting president accused Russia of open aggression on Friday and said Moscow was following a similar scenario to the one before it went to war with Georgia in 2008.

Urging President Vladimir Putin to stop "provocations" in Crimea, Oleksander Turchinov drew a comparison to Russia's intervention in Georgia over the breakaway Akhazia region which has a large ethnic Russian population.

"Russia has sent forces into Crimea ... they are working on scenarios which are fully analogous with Abkhazia, when having initiated a military conflict, they started to annex the territory," he said in televised comments.

Ukraine protested to Russia that it had violated its airspace and broken the terms of an agreement under which Moscow leases a base for its Black Sea fleet in Crimea.

The Foreign Ministry gave no details but the Ukrainian border guard service said more than 10 Russian military helicopters had flown from Russia into Ukrainian airspace over the Crimea region.

Video purportedly showing 12 Russian Mi-24 and Mi-8 helicopters in Crimea

The Russian Foreign Ministry said movements of armored vehicles belonging to the Russian Black Sea Fleet were prompted by the need to ensure security of its base and didn't contradict the lease terms.

Any Russian military incursion in Crimea would dramatically raise the stakes in Ukraine's conflict, which saw pro-Russian President Viktor Yanukovych flee last weekend after three months of anti-government protests. Yanukovych vowed Friday at a news conference in Russia to "keep fighting for the future of Ukraine," though he called any military action "unacceptable."

Moscow has vowed to protect Russian-speaking Ukrainians in Crimea, where it has a major naval base, and Ukraine and the West have warned Russia to stay away. Russia did not confirm its troops were involved in Friday's action in Crimea, which would be a major escalation.

The United States warned Russia on Friday not to inflame the situation in Ukraine. The White House said in a statement that Russian intervention in the Ukraine would be a "grave mistake," adding that Ukraine's territorial integrity must be respected.

Speaking at a news conference with the foreign minister of Colombia, U.S. Secretary of State John Kerry said he had raised the issue of the airports as well as reports of Russian armored vehicles and personnel in Ukraine with Russian Foreign Minister Sergei Lavrov.

"While we were told that they are not engaging in any violation of the sovereignty and do not intend to, I nevertheless made it clear that that could be misinterpreted at this moment and that there are enough tensions that it is important for everybody to be extremely careful not to inflame the situation and not to send the wrong messages," Kerry said.

"The question is whether or not what is happening now might be crossing a line in any way and we are going to be very careful in making our judgments about that," he added.

In Kiev, Ukraine's parliament adopted a resolution demanding that Russia halt steps it says are aimed against Ukraine's sovereignty and territorial integrity, and called for a UN Security Council meeting on the crisis.

"I can only describe this as a military invasion and occupation," Ukraine's newly named interior minister, Arsen Avakov, wrote in a Facebook post.

The chief of Ukraine's security council, Andriy Parubiy, seemed to strike a less strident tone later in the day, saying gunmen had tried to "seize" the airports in the Crimean cities of Simferopol and Sevastopol but insisting in comments to the Interfax news agency that "de-facto the airports are controlled by the law enforcement bodies of Ukraine."

Ukraine's State Border Guard Service also said about 30 Russian marines from Russia's Black Sea Fleet — which is based in Sevastopol — had taken up position outside the Ukrainian Coast Guard base in the area. It said the marines said they were there to prevent any weapons at the base from being seized by extremists.

Russia's defense ministry had no comment.

Yanukovych made his first public appearance since fleeing Ukraine in the southern Russian city of Rostov-on-Don, not far from the Ukrainian border. It was the first confirmation that he had left the country, and he said he was "forced" to do so only after his family received threats.

"I intend to keep fighting for the future of Ukraine," he said.

Yanukovych said he supports Crimea's residents who are worried about "nationalists" in Kiev and added that Russia cannot stand by while events in Ukraine unfold. He denied, however, that this amounts to a call for military intervention.

"Any military action in this situation is unacceptable," he said.

The prosecutor-general's office in Kiev said it started moves to seek Yanukovych's extradition to Ukraine, where he is wanted on suspicion of mass murder in last week's violent clashes between protesters and police, during which over 80 people were killed.

Associated Press journalists approaching the Sevastopol airport found the road leading up to it blocked by two military trucks and a handful of gunmen wearing camouflage uniforms and carrying assault rifles.

A car with Russian military plates was stopped at the roadblock. A man wearing a military uniform with a Russian flag on his sleeve got out of the car and was allowed to enter on foot after a brief discussion with the gunmen.

At the airport serving Simferopol, commercial flights were landing and taking off despite dozens of armed men in military uniforms without markings patrolling with assault rifles. They didn't stop or search people leaving or entering the airport, and refused to talk to journalists.

One man who identified himself only as Vladimir said the men were part of the Crimean People's Brigade, which he described as a self-defense unit ensuring that no "radicals and fascists" arrive from other parts of Ukraine. There was no way to verify his account.

The airport deployments came a day after masked gunmen with rocket-propelled grenades and sniper rifles seized the parliament and government offices in Simferopol and raised the Russian flag. Ukrainian police cordoned off the area but didn't confront the gunmen. They remained in control of the buildings Friday.

The Russian foreign and defense ministries had no comment. Russia's state RIA Novosti and Interfax cited an unnamed official from the Russian Black Sea Fleet denying involvement, saying Russian servicemen stationed in Crimea have not moved into the airports and denying that the Russian military was in control there.

Tensions between the two countries were high, however. Russia continued with massive combat readiness exercises involving most of its troops in western and southern Russia that it said were unrelated to the Ukraine conflict. The moves were reminiscent of Cold War brinksmanship.

The Kremlin, in a statement published late Thursday, said President Vladimir Putin had instructed the government to "maintain contacts with the counterparts in Kiev in what concerns trade and economic ties between Russia and Ukraine."

Moscow has been sending mixed signals about Ukraine but pledged to respect its territorial integrity. Putin has long dreamed of pulling Ukraine, a country of 46 million people considered the cradle of Russian civilization, closer into Moscow's orbit.

Meanwhile, Swiss prosecutors announced they had launched a criminal investigation against Yanukovych and his son Aleksander over "aggravated money laundering." They said police and Geneva's chief prosecutor conducted a search and seized documents Thursday at the premises of a company owned by Aleksander Yanukovych.

Switzerland and Austria both said they would freeze any assets Yanukovych and his entourage might have in those countries.

Ukraine's population is divided in loyalties between Russia and the West, with much of western Ukraine advocating closer ties with the European Union while eastern and southern regions look to Russia for support.

Crimea, a southeastern peninsula of Ukraine that has semi-autonomous status, was seized by Russian forces in the 18th century under Catherine the Great, and was once the crown jewel in Russian and then Soviet empires.

It became part of Ukraine in 1954 when Soviet leader Nikita Khrushchev transferred jurisdiction from Russia, a move that was a mere formality until the 1991 Soviet collapse meant Crimea landed in an independent Ukraine.

In a bid to shore up Ukraine's fledgling administration, the International Monetary Fund has said it is "ready to respond" to Ukraine's bid for financial assistance; Ukraine's finance ministry has said it needs $35 billion over the next two years to avoid default.

The European Union is also considering emergency loans for a country that is the chief conduit of Russian natural gas to western Europe.

And Putin, in his statement, asked his government to "hold consultations with foreign partners including the IMF and the G8 nations to provide financial aid to Ukraine."

Open gallery view A soldier atop a Russian armored personnel carriers with a road sign reading "Sevastopol - 32 kilometers, Yalta - 70 kilometers", near the town of Bakhchisarai, Ukraine, Feb. 28, 2014. Credit: AP

Open gallery view Russian armored personnel carriers and a truck are parked on the side of the road near the town of Bakhchisarai, Ukraine, Feb. 28, 2014. Credit: AP