Ukraine's acting president has ordered the country's armed forces on full readiness because of the threat of "potential aggression".
Speaking live on Ukrainian TV, Oleksandr Turchynov said he had also ordered security to be stepped up at nuclear power plants, airports and other strategic infrastructure.
His comments came after Russia's parliament approved military intervention in Ukraine and Russian troops and pro-Russian units took up positions across the strategic Crimea region.
The unanimous vote in the Russian parliament formalised what Ukrainian officials described as an ongoing deployment of Russian troops in the strategic region of Crimea.
It also raised the possibility that Moscow could send its military elsewhere in Ukraine.
The Russian parliament's backing of President Vladimir Putin came as pro-Russian demonstrations broke out in Ukraine's Russian-speaking east, where protesters raised Russian flags and beat up supporters of the new Ukrainian government.
Russia's move sharply raised the stakes in the conflict following the ousting of Ukraine's pro-Russian president last week by a protest movement aimed at turning Ukraine towards the European Union and away from Russia.
President Barack Obama warned Moscow on Friday "there will be costs" if Russia intervenes militarily.
In Crimea, the pro-Russian regional prime minister had earlier claimed control of the military and police there and asked Mr Putin for help in keeping peace, sharpening the discord between the two neighbouring Slavic countries.
Armed men described as Russian troops took control of key airports and a communications centre 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 intervene on the 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.
Crimea'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.
Ukrainian 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," Mr Yatsenyuk was quoted as saying by the Interfax news agency. "Russian partners, stop provoking civil and military resistance in Ukraine."
Russia put pressure on Ukraine from another direction when a spokesman for state gas company Gazprom said that Ukraine owed 1.59 billion US dollars in overdue bills for imported gas.
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 break-up in 1991 meant Crimea landed in an independent Ukraine.
Russia has taken a confrontational stance towards its southern neighbour after pro-Russian president Viktor Yanukovych fled the country. Mr 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. Mr Yanukovych took refuge in Russia and still says he is president.
Mr Aksyonov, the head of the main pro-Russia party on the peninsula, appealed to Mr Putin "for assistance in guaranteeing peace and calmness on the territory of the autonomous republic of Crimea".
Mr Aksyonov was voted in by the Crimean parliament on Thursday 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.
Mr Obama called on Russia to respect the independence and territory of Ukraine and not try to take advantage of its neighbour, which is undergoing political upheaval.
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.
Meanwhile, flights remained halted from Simferopol's airport. Dozens of armed men in military uniforms without markings patrolled the area. They did not stop or search people leaving or entering the airport, and refused to talk to journalists.
Barack Obama called on Russia to de-escalate tensions by pulling its forces back to bases in the Crimean peninsula and to refrain from any interference elsewhere in Ukraine.
The White House said Mr Obama delivered that message to Mr Putin during a 90-minute telephone call today.
A statement from the White House said the US condemed Russia's military reach into Ukrainian territory.
Russia says Mr Putin emphasised that real threats exist to the life and health of Russian citizens and that Russia had the right to protect its interests and the Russian-speaking people who lived in that part of Ukraine.
Russia's parliament on Saturday gave Putin the right to deploy the Russian military in Ukraine.