The pro-Russian leader of Ukraine's Crimea region claimed control of the military and police there and appealed to Russia's President Vladimir Putin for help in keeping peace, sharpening the discord between the two Slavic neighbor countries.
It was the latest escalation following the ouster of Ukraine's pro-Russian president last week by a protest movement aimed at turning Ukraine toward the European Union and away from Russia.
Armed men described as Russian troops took control of key airports and a communications center in Crimea on Friday.
Ukraine has accused Russia of a "military invasion and occupation" - a claim that brought an alarming new dimension to the crisis, and raised fears that Moscow is moving to annex a strategic peninsula where Russia's Black Sea fleet is based.
Ukraine's population is divided in loyalties between Russia and Europe, with much of western Ukraine advocating closer ties with the European Union while eastern and southern regions look to Russia for support. Crimea is mainly Russian-speaking.
Crimean's prime minister, Sergei Aksyonov, declared that the armed forces, the police, the national security service and border guards in the region will answer only to his orders.
Ukraine's Prime Minister Arseny Yatsenyuk opened a cabinet meeting in the capital, Kiev, by calling on Russia not to provoke discord in Crimea, a peninsula on the Black Sea.
"We call on the government and authorities of Russia to recall their forces, and to return them to their stations," Yatsenyuk was quoted as saying by the Interfax news agency.
"Russian partners, stop provoking civil and military resistance in Ukraine."
Crimea only became part of Ukraine in 1954 when Soviet leader Nikita Khrushchev transferred jurisdiction from Russia, a move that was a mere formality when both Ukraine and Russia were part of the Soviet Union. The Soviet breakup in 1991 meant Crimea landed in an independent Ukraine.
President Barack Obama warned Moscow on Friday "there will be costs" if it intervenes militarily.
Russia has taken a confrontational stance toward its southern neighbor after pro-Russian President Viktor Yanukovych fled the country.
Yanukovych was voted out of office by parliament after weeks of protests ended in violence that left over 80 people dead.
Demonstrators sought his resignation after he backed out of signing an agreement to bring Ukraine closer to the European Union instead of Russia. Yanukovych took refuge in Russia and still says he's president.
Aksyonov, the head of the main pro-Russia party on the peninsula, appealed to Putin "for assistance in guaranteeing peace and calmness on the territory of the autonomous republic of Crimea."
Aksyonov was voted in by the Crimean parliament after pro-Russia gunmen seized the building and as tensions soared over Crimea's resistance to the new authorities in Kiev, who took office this week.
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.
He said such action by Russia would represent a "profound interference" in matters he said must be decided by the Ukrainian people.
"The United States will stand with the international community in affirming that there will be costs for any military intervention in Ukraine," he said. He did not say what those costs might be.
Poland called for an end to "provocative movements of troops on the Crimean Peninsula,"
"Any decisions that will be taken in the coming days, including of military nature, could have irreparable consequences for the international order," the Polish foreign ministry said in a statement.
Swedish Foreign Minister Carl Bildt wrote on Twitter that it was "obvious that there is Russian military intervention in Ukraine. Likely immediate aim is to set up puppet pro-Russian semi-state in Crimea."
At the United Nations, the Ukrainian ambassador, Yuriy Sergeyev, said Friday that 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.
Russia has kept silent on claims of military intervention and has said any troop movements are within agreed rules, even as it maintained its hard-line stance on protecting ethnic Russians in Crimea.
Meanwhile, flights remained halted from Simferopol's airport. Dozens of armed men in military uniforms without markings patrolled the area. 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 independently verify his account.