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 Barack 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.
An unidentified armed man patrols a square in front of the airport in Simferopol, Ukraine, on Friday. Dozens of armed men in military uniforms without markings occupied the airport in the capital of Ukraine's strategic Crimea region early Friday.
"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."
He did not say what those costs might be.
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.
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.
In Kiev, Ukraine's newly named interior minister accused Russia of military aggression.
"I can only describe this as a military invasion and occupation," Arsen Avakov wrote in a Facebook post.
In recent conversations between U.S. and Russian officials, including a lengthy telephone conversation between Obama and Putin just last week, Obama said the U.S. has made clear that Russia can be part of an international community's effort to support the stability and success of Ukraine.