The White House says President Barack Obama is getting regular updates on the situation in Ukraine.
Obama's national security team is discussing Ukraine at a White House meeting Saturday, the eve of a vote on whether Ukraine's Russia-leaning Crimea region should secede and seek annexation by Moscow. Obama and the leaders of other Western nations have denounced the vote as illegal and say they won't recognize the results.
Earlier Saturday at the United Nations, Russia vetoed a U.S.-sponsored resolution declaring Sunday's referendum illegal.
Crimea became a flashpoint after Ukraine's pro-Russia president fled the country last month after months of anti-government protests.
Russia refuses to recognize Ukraine's new government and has effectively taken control of the Crimea region.