President Barack Obama said Friday he was deeply concerned about reports of Russian military activity in Ukraine, while a US military official said Moscow had sent several hundreds troops into Crimea.
Obama made a hurriedly scheduled statement at the White House after a Ukrainian official warned of a Russian "invasion" and said up to 2,000 Russian troops had arrived in the restive peninsula.
"We are now deeply concerned by reports of military movements taken by the Russian Federation inside of Ukraine," Obama said.
As US concern mounted over the situation spiralling out of control after the ouster of the pro-Moscow government in Kiev, Obama did not personally confirm Russian troops had touched down in Crimea.
But a military official told AFP after the president's statement that "it looks like they've moved several hundred troops there."
The official said that the Kremlin had given no advance warning of the movement.
Obama recognized that Russia had interests and cultural and economic ties with Ukraine, and also had a military facility in Crimea, which was ceded to the Soviet republic of Ukraine by the Soviet Union in 1954.
But he said any violation of Ukraine's sovereignty and territorial integrity would be "deeply destabilizing."
"The United States will stand with the international community in affirming that there will be costs for any military intervention in Ukraine," Obama said.
He also warned that a Russian military intervention in the post-Soviet state would "represent a profound interference in matters that must be determined by the Ukranian people."
Obama said that Vice President Joe Biden had spoken Friday with the new prime minister of Ukraine Arseniy Yatsenyuk to offer Washington's support for his country's democratic future.
He said the US government would continue to communicate directly with the Russian government on the crisis.
Earlier Washington's envoy to the United Nations Samantha Power called for urgent international mediation in Crimea and for Russia to withdraw forces it was building up in the region.
"The United States calls on Russia to pull back the military forces that are being built up in the region, to stand down and to allow the Ukrainian people the opportunity to pursue their own government, create their own destiny and to do so freely without intimidation or fear," Power said after an emergency Security Council meeting.
"The United States calls for an urgent, international mediation mission to the Crimea to begin to de-escalate the situation and facilitate productive and peaceful dialogue among all Ukranian parties."
Also on Friday, US Secretary of State John Kerry spoke by telephone with his Russian counterpart Sergei Lavrov for the fourth time in seven days.
The call came after Ukrainian authorities said they had regained control of two airports in southern Crimea which they said were seized during an "armed invasion" by Russian forces.
But it took place before Ukraine's interim president Oleksandr Turchynov appealed to President Vladimir Putin to stop Russia's "naked aggression" against the country and withdraw from the flashpoint Crimea peninsula.