The US president has warned Russia's head of state against military action in Ukraine after its new leaders accused it of deploying troops in Crimea.
US President Barack Obama has warned Russia against any military intervention in Ukraine after the country's new leaders accused Moscow of deploying forces in the Crimea region.
A week after Russian-backed President Viktor Yanukovich was ousted in Kiev, armed men took control of two airports in Crimea on Friday in what Kiev described as an invasion and occupation by Moscow's forces in a region with an ethnic Russian majority.
Acting President Oleksander Turchinov said Russia, which has a naval base in Crimea, was following a scenario like the one before it went to war with fellow former Soviet republic Georgia in 2008 over two breakaway regions.
The crisis, which began after Yanukovich triggered protests by spurning a political and trade deal with the European Union, is stoking tensions in a geopolitical battle between East and West that has echoes of the Cold War.
"We are now deeply concerned by reports of military movements taken by the Russian Federation inside of Ukraine," Obama told reporters in Washington.
"The United States will stand with the international community in affirming that there will be costs for any military intervention in Ukraine."
Any violation of Ukraine's sovereignty and territorial integrity would be "deeply destabilising," he said.
Obama and European leaders would consider skipping a G8 summit this summer in the Russian city of Sochi if Moscow intervened militarily, a senior US official said.
The G8 includes the world's seven leading industrial nations and Russia, and Russian President Vladimir Putin considers hosting such events as a way to show how far Russia has come since the Soviet Union collapsed in 1991.
Washington's relations with Moscow are already cool because of differences over the conflict in Syria, Putin's record on human rights and Russia's decision to harbour former US spy agency contractor Edward Snowden.
Russia's ambassador to the United Nations, Vitaly Churkin, dismissed the criticism, saying any movements of its forces in Crimea were in line with agreements with Ukraine.
Gunmen have taken over the regional parliament in Crimea, and control the main international airport and a military airfield on the strategic Black Sea peninsula.
A representative of Acting President Turchinov said 13 Russian aircraft had landed with 150 personnel on each plane. The Ukrainian leadership also said more than 10 Russian military helicopters flew over Crimea, and Russian servicemen blockaded a unit of the Ukrainian border guard near the port city of Sevastopol, home to Russia's Black Sea fleet.
A local television station reported that another military aerodrome had been taken over by armed men overnight, but the report was not independently confirmed.
Phone lines have been severed in some areas and witnesses say they have seen armoured personnel carriers on the move.
There has been no bloodshed and no military clashes despite a warning by Ukraine's Defence Ministry that "radical forces" planned to disarm Ukrainian military units in Crimea.
Ukraine's leaders say about 100 people were killed, some of them by police snipers, during protests in the Ukrainian capital Kiev that began last November.
Russia cannot be indifferent
Yanukovich (63) resurfaced in southern Russia on Friday after a week on the run, defiantly telling a packed room of journalists that he was still leader of the sprawling former Soviet republic of 46 million.
He said he had not ordered police to open fire on the protesters in Kiev and that Russia should use all means at its disposal to stop what he described as chaos in Ukraine.
"Russia cannot be indifferent, cannot be a bystander watching the fate of as close a partner as Ukraine," Yanukovich told a news conference. "Russia must use all means at its disposal to end the chaos and terror gripping Ukraine."
He said he had not seen Putin since fleeing to Russia but had spoken to him by telephone and was surprised the Russian leader was not more vocal on the crisis.
Putin has said nothing in public about the crisis since Yanukovich was ousted a week ago.
A Kremlin statement offered conciliatory remarks about international cooperation over heavily indebted Ukraine but Russian officials have blamed the crisis on the West and accused it of meddling in what Moscow considers its back yard. Loss of influence in Ukraine is a bitter blow for Putin.
Co-operation over financial assistance
The Russian Foreign ministry said on its Facebook page that Russia's Consulate General in Crimea would hand out Russian passports to the servicemen of Ukraine's now-disbanded Berkut riot police. Protesters had accused the Berkut of firing the live bullets that killed dozens of protesters in Kiev.
Moscow has also promised to defend the interests of its citizens in Ukraine. It has said it will not intervene by force, but its rhetoric has echoed the run-up to its war in Georgia, where it said ethnic Russians in the breakaway regions of Abkhazia and South Ossetia needed protection.
Any armed confrontation in Crimea would have major global repercussions.
Moves are under way, however, to prop up Ukraine's faltering economy. The new Ukrainian leadership has said the country needs about $35-billion over the next two years to stave off bankruptcy. It said on Friday it hoped to get financial aid soon and was prepared to fulfill the reform criteria of the International Monetary Fund, which will visit Kiev next week.
The fate of a $15-billion Russian bailout package is unclear, with only about $3-billion of it released so far.