US President Barack Obama has warned Russia of "costs" if it got involved militarily in the Ukrainian crisis.
Since last week's toppling of President Yanukovych, tensions have escalated in Russian-majority Crimea. Reports suggest around 2,000 Russian troops have moved into the peninsula as the acting Ukrainian President Oleksander Turchynov accused Russia of trying to provoke an armed conflict.
Obama commended the interim government for showing 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,
"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."