SEVASTOPOL, Ukraine — Armed men described as Russian troops took control of key airports in Crimea on Friday and Russian transport planes flew into the strategic region, Ukrainian officials said, an ominous sign of the Kremlin’s iron hand in Ukraine. President Obama bluntly warned Moscow “there will be costs” if it intervenes militarily.

The sudden arrival of men in military uniforms patrolling key strategic facilities prompted Ukraine to accuse Russia of a “military invasion and occupation” – a claim that brought an alarming new dimension to the crisis.

In a hastily arranged statement delivered from the White House, Obama called on Russia to respect the independence and territory of Ukraine and not try to take advantage of its neighbor, which is undergoing political upheaval.

“Any violation of Ukraine’s sovereignty and territorial integrity would be deeply destabilizing,” Obama said.

Such action by Russia would not serve the interests of the Ukrainian people, Russia or Europe, Obama said, and would represent a “profound interference” in matters he said must be decided by the Ukrainian people.

“Just days after the world came to Russia for the Olympic Games, that would invite the condemnation of nations around the world,” Obama said. “The United States will stand with the international community in affirming that there will be costs for any military intervention in Ukraine.”

UKRAINIAN PRESIDENT RESURFACES

Earlier Friday, Ukraine’s fugitive president resurfaced in Russia to deliver a defiant condemnation of what he called a “bandit coup.”

Appearing for the first time since fleeing Ukraine last week, Viktor Yanukovych struck a tone both of bluster and caution – vowing to “keep fighting for the future of Ukraine,” while ruling out seeking Russian military help.

“Any military action in this situation is unacceptable,” Yanukovych told reporters in the southern city of Rostov-on-Don, near the border with Ukraine. Then, seeking to make a firm point, he tried – and failed – to break a pen.

At the United Nations, the Ukrainian ambassador, Yuriy Sergeyev, said that 10 Russian transport aircraft and 11 attack helicopters had arrived in Crimea illegally, and that Russian troops had taken control of two airports in Crimea.

He described the gunmen posted outside the two airports as Russian armed forces as well as “unspecified” units.

“Some of them identified themselves as Russians. We know specifically some of the units,” Sergeyev said. He also said the Russians had captured the main air traffic control center on Crimea.

TRANSPORT PLANES LAND

Serhiy Astakhov, a spokesman for the Ukrainian border service, said eight Russian transport planes landed in the Crimea Peninsula with unknown cargo.

He told The Associated Press that the Il-76 planes arrived unexpectedly and were given permission to land, one after the other, at Gvardeiskoye air base, north of the regional capital, Simferopol. Astakhov said the people in the planes refused to identify themselves and waved off customs officials, saying they didn’t require their services.

Russia kept silent on claims of military intervention, even as it maintained its hard-line stance on protecting ethnic Russians in Crimea, a territory that was once the crown jewel in Russian and then Soviet empires and has played a symbolic role in Russia’s national identity.

Earlier Friday, AP journalists in Crimea spotted a convoy of nine Russian armored personnel carriers on a road between the port city of Sevastopol, where Russia has a naval base, and the regional capital, Simferopol. Later in the day, the airspace was closed over the peninsula, apparently due to tensions at the two airports.

Russian armored vehicles bearing the nation’s tricolor rumbled across Crimea and men described as Russian troops took position at airports and a coast guard base.

Oleksandr Turchynov, who stepped in as president after Yanukovych fled Kiev last weekend, urged Russian President Vladimir Putin to stop “provocations” in Crimea and pull back military forces from the peninsula. Turchynov said the Ukrainian military would fulfill its duty but would not be drawn into provocations.

POSSIBLE U.S. RESPONSE

Kerry and White House spokesman Jay Carney both said any Russian military intervention would be a grave mistake and that the United States was watching closely. They did not spell out any consequences for an intervention.

U.S. Ambassador to the U.N. Samantha Power told reporters that the United States was proposing an urgent mediation mission to help resolve Ukraine’s crisis.

The Russian armored personnel vehicles seen by AP reporters 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 Black Sea Fleet naval base it maintains in Sevastopol under a lease agreement with Ukraine.

The Russian Foreign Ministry said the military vehicles were deployed to ensure the security of its base and didn’t contradict the lease terms.

A duty officer at the Ukrainian Defense Ministry said it had no information about the vehicles’ movements.

AP journalists approaching the Sevastopol airport found the road leading 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.

Meanwhile, Ukraine International Airlines said it had canceled flights to and from the Simferopol airport on Friday evening and Saturday because of the closure of the airspace over Crimea.