White House senior adviser Dan Pfeiffer said the Obama administration's top priority is supporting the new Ukrainian government "in every way possible."
He also said the United States would not recognize the results of a referendum taking place in Crimea today on whether it should become part of Russia.
Pfeiffer said everything that Russia has done in Crimea has been a violation of international law and bad for stability in the region.
Members of Congress said they were prepared to enact tough sanctions on various Russian leaders. It's also considering action to help the Ukrainian economy, but USD 1 billion in loan guarantees is on hold because Congress is on a break.
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"President Putin has started a game of Russian roulette and I think the United States and the West have to be very clear in their response because he will calculate about how far he can go," said Sen. Robert Menendez, the Democratic chairman of the Senate Foreign Relations Committee.
"Putin will continue to do this. He did it in Georgia a few years ago. He's moved into Crimea and he will move into other places unless we show that long-term resolve."
Democratic Sen. Chris Murphy returned early today from meetings in Ukraine. He called an annexation vote taking place in Crimea a "sham referendum.