Barack Obama warned Russia that the West will be forced to apply a cost to Moscow if it fails to change course in its dispute with Ukraine.
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 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.
US President Barack Obama held a second Ukraine-focused meeting of the National Security Council in two days.
Mr Obama met with the council for more than two hours on Monday evening and again on Tuesday morning to discuss steps the US can take to de-escalate the crisis in Crimea.
He said that Russia's legitimate interests in Ukraine did not give it a right to "use force as a means of exerting influence", and insisted the Ukrainian people should be able to decide their own future.
The US President Barack Obama said "Russia's actions in Ukraine is not a sign of strength, but a signal that neighbouring countries have deep concerns about Russian meddling."
"President Putin seems to have a different set of lawyers making a different set of interpretations," Mr Obama said. "But I don't think that's fooling anybody."
Speaking in Washington, Mr Obama said that if Ukraine has legitimate election, there "should be no question that Ukrainian people are capable of governing themselves."
The White House has released an image of President Barack Obama's meeting with the National Security Council to discuss the situation in Ukraine.
US President Barack Obama met with his national security team for more than two hours to discuss ways to "further isolate" Russia, a White House official said.
The group talked about ways to "reinforce that the Russians still have an opportunity to take immediate steps to de-escalate the situation or they face further political and economic repercussions from the international community," Reuters reported, citing the unnamed official.
US President Obama said Russia had violated international law in its military intervention in Ukraine.
Speaking before he met with Israeli Prime Minister Benjamin Netanyahu, he said:
"I spent the weekend talking to leaders across Europe and I think the world is broadly united in recognising that the steps Russia has taken, are a violation of Ukraine's sovereignty, Ukraine's territorial integrity and international law."
"We are strongly supportive of the interim Ukrainian government.[...] Obviously the facts on the ground are deeply troubling, and Russia has a large army that borders Ukraine, but what is also true is that over time this will be a costly proposition for Russia."
Barack Obama said Russia had violated international law in its military intervention in Ukraine.
He also said the US government has warned it will look at a series of economic and diplomatic sanctions that would isolate Moscow. He added:
Over time this will be a costly proposition for Russia.
And now is the time for them to consider whether they can serve their interests in a way that resorts to diplomacy as opposed to force.
President Obama spoke for 90 minutes with President Putin about the situation in Ukraine, the White House said.
Obama told Putin that Russia's actions in Ukraine, "would negatively impact Russia’s standing in the international community."
President Obama expressed his "deep concern over Russia’s clear violation of Ukrainian sovereignty and territorial integrity, which is a breach of international law," the White House said.
The United States called on Russia to de-escalate tensions by withdrawing its forces back to bases in Crimea and to refraining from any interference elsewhere in Ukraine.
The Kremlin released their own version of the call, saying Putin asserted that Moscow "reserves the fight" to protect its interests and "drew attention" to the "provocative, criminal actions" of "ultra-nationalists."
In a phone conversation with President Putin US President Obama expressed concern over Russia's "clear violation of Ukrainian sovereignty and territorial integrity."
Obama said that the US will suspend participation in G8 preparatory meetings in Sochi and warned Russia could face "greater political and economic isolation."