U.S. President Barack Obama has condemned Russia saying it is on "the wrong side of history" for mobilizing forces around Ukraine's Crimean peninsula.
Mr. Obama told reporters Monday that the United States is considering a series of economic and diplomatic steps "that will isolate Russia."
"I think the world is largely united in recognizing that the steps Russia has taken are a violation of Ukraine's sovereignty, Ukraine's territorial integrity, that there a violation of international law. There a violation of previous agreements that Russia has made with respect to how it treats and respects its neighbors, and as a consequence we got strong statements from NATO, from the G7, condemning the actions that Russia has taken."
Mr. Obama called on the U.S. Congress to work on an aid package to assist Ukraine.
U.S. Secretary of State John Kerry is heading to Ukraine in an effort to defuse the crisis between Kyiv and Moscow.
During a joint press conference Monday in Washington with visiting Moldovan Prime Minister Iurie Leanca, Kerry said Russia has used its influence to apply pressure on Ukraine similar to tactics used in Moldova.
"Russia, in some of the challenges we are seeing right now in Ukraine, has put pressure on Moldova. There are challenges with respect to their energy sources and also their ability to trade."
Earlier, the White House says U.S. Vice President Joe Biden urged Russian Prime Minister Dmitry Medvedev, in a telephone call, to withdraw his forces from Ukraine, support the immediate deployment of international monitors to Ukraine and begin a dialogue with the Ukrainian government.
Russian media quoted Mr. Medvedev's press secretary as saying that the call was initiated by the American side and that the Russian prime minister stressed the need to "protect Ukrainian citizens, including in Crimea, as well as citizens of the Russian Federation located on the territory of Ukraine."
Republican Senator John McCain told a meeting of the American Israel Public Affairs Committee on Monday that Russia's planned takeover of Crimea is a "blatant act" and one that must be unacceptable to the world community.
"But, the fact is that Crimea is a sovereign part of the sovereign nation of Ukraine and the people of Ukraine by the hundreds of thousands went to a square in sub-freezing weather, saying that they did not want to be part of Putin's Russia, and that's what it was all about."