The U.S. on Monday accused Russia of instigating the storming of government offices in eastern Ukraine, unrest that echoed the events preceding Russia's annexation of Crimea.
There is evidence supporting the contention of Ukrainian officials that some of the pro-Russian separatists who seized administration buildings in the cities of Luhansk and Donetsk were paid provocateurs brought in from outside, White House press secretary Jay Carney said in Washington.
"If Russia moves into eastern Ukraine, either overtly or covertly, this would be a very serious escalation," Carney said. There "will be costs for further transgressions" against Ukraine's sovereignty, he added.