The Cold War has returned with a vengeance, judging by the mood in Washington with talk of extensive sanctions against Russia.
President Barack Obama expressed concern about reported Russian military movement inside crisis-torn Ukraine and warned of consequences.
US president Barack Obama and vice-president Joe Biden have joined forces in an exercise video encouraging Americans to get fit.
US President Barack Obama will meet with Ukrainian Prime Minister Arseny Yatseniuk at the White House on Wednesday.
The White House called the meeting, which comes just days before a referendum on secession by Crimea, a "show of support for Ukrainian sovereignty."
The leaders are set to talk about "how to find a peaceful resolution to Russia's ongoing military intervention in Crimea that would respect Ukrainian sovereignty and territorial integrity," the White House said.
President Obama made a series of phone calls on Saturday to world leaders about the ongoing crisis in Ukraine, including Prime Minister David Cameron, French President Francois Hollande, and Italian Prime Minister Matteo Renzi, the White House said.
He also held a conference call about the situation with the presidents of Lithuania, Latvia and Estonia, White House spokesman Josh Earnest said in a statement.
President Barack Obama has urged Russian President Vladimir Putin to accept the terms of a potential diplomatic solution to the Ukraine crisis.
In their second phone conversation in the past six days, Obama outlined the terms of a diplomatic "off-ramp" that US officials are promoting.
Under terms of the deal, Russia would pull back troops to bases in Crimea, allow international monitors in to ensure the rights of ethnic Russians are respected and consent to direct talks with Ukraine officials.
"President Obama indicated that there is a way to resolve the situation diplomatically," the White House said in a statement.
US President Barack Obama spoke to Russian President Vladimir Putin for an hour to discuss the situation in Ukraine.
"President Obama emphasised that Russia’s actions are in violation of Ukraine’s sovereignty and territorial integrity, which has led us to take several steps in response, in coordination with our European partners," the White House said.
Barack Obama has condemned a move by Ukraine's Crimea region to set up a referendum to join Russia.
"The proposed referendum on the future of Crimea would violate the Ukrainian constitution and violate international law," the US President said.
Mr Obama said "any discussion about the future of Ukraine must include the legitimate government of Ukraine".
He added: "In 2014, we are well beyond the days when borders can be redrawn over the heads of democratic leaders."
US President Barack Obama said if the "violation of international law continues, the resolve of the United States, our allies and the international community will remain firm" in response to Russian actions in Ukraine.
President Barack Obama has said Russia should allow international monitors into all parts of Ukraine including Crimea to ensure the rights of all Ukrainians are being "respected".
Mr Obama confirmed that he had signed an executive order that authorised sanctions against individuals who "violated the sovereignty and territorial integrity" of Ukraine.
US President Barack Obama has ordered sanctions on individuals and entities responsible for undermining democratic institutions in Ukraine, the White House said.
The US State Department is putting together visa bans on some individuals involved in threatening territorial integrity of Ukraine.
The visa bans apply to both Russian and Ukrainian officials and other individuals, a senior US official said.