WASHINGTON - President Barack Obama told Russian President Vladimir Putin on Sunday that Crimea's vote to secede from Ukraine and join Russia would never be recognized by the United States.
The two leaders spoke after residents in Crimea voted overwhelmingly in favor of the split.
The White House said Obama told Putin the Crimean vote violates the Ukrainian constitution and occurred under duress of Russian military intervention.
He also said the U.S. was prepared to impose retaliatory penalties on Russia.
Obama urged Putin to work with Ukraine as well as Western nations to resolve the crisis diplomatically. Obama also asked Putin to support the deployment of international monitors to help prevent violence in Ukraine as that country prepares for spring elections.
Even before official results were announced, the White House denounced the vote, saying "no decisions should be made about the future of Ukraine without the Ukrainian government" and noting that Russia had rejected the deployment of international monitors in Crimea to ensure the rights of ethnic Russians there were protected.
"Russia has spurned those calls as well as outreach from the Ukrainian government and instead has escalated its military intervention into Crimea and initiated threatening military exercises on Ukraine's eastern border," the White House said.
"Russia's actions are dangerous and destabilizing," the White House said.
U.S. officials reaffirmed that the Obama administration will, along with the European Union, impose penalties on Russia if it annexes the strategic region. They also warned that any Russia moves on east and south Ukraine would be a grave escalation requiring additional responses.
Secretary of State John Kerry called on Moscow to return its troops in Crimea to their bases, pull back forces from the Ukraine border, halt incitement in eastern Ukraine and support the political reforms in Ukraine that would protect ethnic Russians, Russian speakers and others in the former Soviet republic that Russia says it is concerned about.
In a call with Russian Foreign Minister Sergey Lavrov, Kerry urged Russia "to support efforts by Ukrainians across the spectrum to address power sharing and decentralization through a constitutional reform process that is broadly inclusive and protects the rights of minorities," the State Department said.
It was their second call since unsuccessful talks Friday in London.
Kerry expressed "strong concerns" about Russian military activities in the southern Ukrainian region of Kherson, just north of Crimea where Russian troops appeared Saturday, and about "continuing provocations" in cities in east Ukraine, the department said.
Kerry "made clear that this crisis can only be resolved politically and that as Ukrainians take the necessary political measures going forward, Russia must reciprocate by pulling forces back to base and addressing the tensions and concerns about military engagement," the department said.
A senior State Department official said Lavrov's willingness to discuss Ukraine political reforms was positive. But the official stressed that the Russian military escalation was of "greatest concern" and must be reversed. The official spoke on condition of anonymity to discuss the private conversation.
White House senior adviser Dan Pfeiffer said that Russia faces penalties that would hurt its economy and diminish its influence in the world if President Vladimir Putin didn't back down. Pfeiffer said the administration was committed to supporting the new Ukrainian government in Kiev "in every way possible."
"President Putin has a choice about what he's going to do here. Is he going to continue to further isolate himself, further hurt his economy, further diminish Russian influence in the world, or is he going to do the right thing?" Pfeiffer said.
U.S. and European officials have said they plan to announce sanctions against Russia, including visa bans and potential asset freezes, on Monday if Putin does not shift course.
But Putin and other Russians have shown no sign they are willing to back down. They insist they will respect the results of the Crimean referendum in which voters in the largely pro-Moscow peninsula are expected to choose joining Russia by a wide margin.
Members of Congress said they were prepared to enact tough sanctions on various Russian leaders, but $1 billion in loan guarantees to help the Ukrainian economy is on hold while Congress is on a break.
"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, D-N.J., chairman of the Senate Foreign Relations Committee.
The top Republican on the committee, Sen. Bob Corker of Tennessee, said the U.S. and Europe were entering a "defining moment" in their relationship with Russia.
"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 of Connecticut, just back from meetings in Ukraine, said Ukrainians he talked to said war could occur if Russia attempts to annex more territory. They indicated that "if Russia really does decide to move beyond Crimea, it's going to be bloody and the fight may be long," Murphy said.
Pfeiffer spoke on NBC's "Meet the Press." Menendez and Corker appeared on "Fox News Sunday." Murphy was on ABC's "This Week."