Washington was working to find out if Moscow has demanded Ukrainian leaders surrender or face an all-out assault.
The reports “if true, in our view constitute a dangerous escalation of the situation, for which we would hold Russia directly responsible,” said State Department spokeswoman Jen Psaki.
Secretary of State John Kerry is due to leave Washington later on Monday to travel to Kiev, in a strong sign of support for the interim leadership in Ukraine which took over after the ousting of president Viktor Yanukovych.
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“He’s going to be discussing, of course, Ukraine’s economic and political needs, seeing what additional support we can provide and really sending a strong message that we support the people of Ukraine, the voices of the people of Ukraine,” Psaki told reporters on a phone conference call.
Kerry had also said last week that he would meet with his Russian counterpart Sergei Lavrov on the sidelines of talks due to be held on Wednesday in Rome.
But with the escalation of tensions in the southern Crimea peninsula over the weekend, Psaki said Kerry’s schedule during his trip to Rome and Paris was not completed yet although she did not rule out a meeting between the two ministers.
“Obviously there’s been a discussion of that, but the schedule’s still being finalized over the course of the days he’ll be in Paris and Rome.”
Washington is also pushing for a meeting of the four signatories of a 1994 Budapest agreement under which Ukraine agreed to give up its Soviet-era nuclear weapons in exchange for guarantees over its sovereignty and territorial integrity.
The United States, Russia, Britain and the Ukraine are all signatories to the treaty.