The White House is telling Russia to stop intervening in eastern Ukraine, or face more sanctions.
The warning from the White House came after groups of pro-Russian demonstrators took over government buildings in at least three cities in eastern Ukraine over the last few days.
At a regular briefing Monday, spokesman Jay Carney said the takeovers are a result of increased Russian pressure on Ukraine. He warned Moscow to make no further moves.
"If Russia moves into eastern Ukraine, either overtly or covertly, this would be a very serious escalation. We call on President Putin and his government to cease all efforts to destabilize Ukraine and we caution against further military intervention," he said.
Carney said there is strong evidence some of the demonstrators were paid, and not local residents.
Pro-Russian demonstrators in the eastern Ukrainian city of Donetsk declared a separate republic, announced a secession referendum, and called for Russia to send in peacekeepers.
The moves resemble events leading up to Russia's annexation of Ukraine's Crimea region last month, and U.S. officials are wary of a repeat.
The White House spokesman indicated the U.S. is prepared to step up sanctions.
"All of this is of concern to us and we've made very clear that should Russia take action that violates Ukraine's territorial integrity further or violates Ukraine's sovereignty further, there will be further consequences," said Carney.
Those consequences could be wider sanctions leveled against entire sectors of the Russian economy, including weapons exports and banking, under executive orders President Obama signed last month.
U.S. officials say they see no evidence that Russia is pulling troops back from the Ukrainian border, where they have been massing for weeks.
Washington says discussions of military action are not on the forefront at this point and has committed to provide non-lethal aid to Ukraine.
U.S. defense officials confirmed the U.S. is sending a ship to the Black Sea in a bid to reassure allies.
Secretary of State John Kerry, meanwhile, has been talking to his Russian counterpart, Sergei Lavrov. The two spoke by phone Monday, and Kerry said he is watching events in Ukraine with "great concern."