President Barack Obama's national security team discussed the Ukraine crisis in a session at the White House on Saturday after a last-ditch bid to find a diplomatic solution to the Cold War-style standoff with Russia floundered.
Secretary of State John Kerry, who just returned from talks with his Russian counterpart in London, was at the White House meeting along with Defense Secretary Chuck Hagel.
Obama did not attend the meeting but was being briefed about it and other developments involving Ukraine, said Laura Lucas Magnuson, a spokeswoman for the White House National Security Council.
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Crimea's pro-Russia parliament has scheduled a referendum on Sunday to decide whether the region should be annexed by Russia, an ominous development that Obama and his national security team have been trying to head off to no avail.
Obama said on Friday he still hopes a diplomatic solution can be found but that the United States and Europe are prepared to impose "consequences" on Russia if it does not loosen its grip on Crimea.
White House deliberations took on a new urgency as Ukraine's military scrambled aircraft and paratroops to confront Russian troops landing on a remote spit of land between Crimea and the mainland.
The Russian move prompted some members of the U.S. Congress to express alarm.
"Russia using provocateurs to provoke violence attacks inside Ukraine to create excuse for invasion of Eastern Ukraine," Republican Florida Senator Marco Rubio said in a Tweet.
Representative Mike Turner, an Ohio Republican who chairs the U.S. delegation to the NATO Parliamentary Assembly, said the fresh Russian move was a deliberate violation of Ukraine's sovereignty and "clearly shows that Russia is an aggressor state."
(Read more: Crimea referendum: Why it's so important)
Kerry held talks with Russian Foreign Minister Sergei Lavrov in London on Friday but they reached no breakthrough in the worst East-West dispute in decades.
A senior U.S. official, speaking on condition of anonymity, said Kerry presented Lavrov with a number of concrete proposals to defuse tensions and address concerns by Moscow over security and protection of minorities within a united Ukraine.
The talks hit a wall early on when it became clear that Lavrov was not authorized by the Kremlin to discuss any proposal that might impact Crimea before the referendum, the official said.
(Read more: $3 billion for Ukraine to go straight to...Russia)
Washington has said it will be ready to move on Monday to impose visa bans and asset freezes, involving Ukrainian and Russian officials, if the referendum goes ahead and it leads to the annexation of Crimea. The European Union and other Western powers have said they will match U.S. sanctions.
Vice President Joe Biden leaves on Monday on a visit to NATO allies Poland and Lithuania to show support for key partners in the region. In Vilnius he will meet the presidents of all three Baltic nations, Lithuania, Estonia and Latvia.