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US President Barack Obama has warned Russia there will be "costs" for any military intervention in Ukraine, the BBC reported.

He said he was deeply concerned by reports of Russian military movements inside the country.

Ukraine's acting president has accused Russia of deploying troops to Crimea and trying to provoke Kiev into "armed conflict.”

Russia's UN ambassador said any troop movements in Crimea were within an existing arrangement with Ukraine.

Speaking from the White House, President Obama commended Ukraine's interim government for its "restraint."

"Any violation of Ukraine's sovereignty and territorial integrity would be deeply destabilising, which is not in the interests of Ukraine, Russia or Europe," he said.

"It would represent a profound interference in matters that must be determined by the Ukrainian people. It would be a clear violation of Russia's commitment to respect the independence and sovereignty and borders of Ukraine - and of international laws."

He added: "Just days after the world came to Russia for the Olympic games, it would invite the condemnation of nations around the world. And, indeed, the United States will stand with the international community in affirming that there will be costs for any military intervention in Ukraine."

Mr Obama did not spell out what any US response might be. However, the BBC's Beth McLeod in Washington says the US is considering exerting economic pressure by withholding the deeper trade ties that Moscow seeks.