In a "gloomy" briefing to senators by senior State Department official Victoria Nuland on Monday night, Nuland outlined the tough sanctions package being prepared by the administration in response to a potential Russian attack, but acknowledged that the US' options to deter an invasion are fairly limited, a person familiar with the briefing said.

It is still unclear whether Russian President Vladimir Putin has made the decision to invade, US officials stressed.

Right now, the Pentagon is planning for a number of different scenarios in the event that an evacuation is necessary, ranging from a smaller evacuation of just nonessential US government employees to a larger one involving a broader swath of American citizens, the sources said.

"We don't know that Putin has made up his mind to use force, but what we do know is that he's putting the Russian military, the Russian security forces in a place where they could act in a pretty sweeping way," CIA Director Bill Burns said at a Wall Street Journal CEO Council Summit on Monday.

The military has similar contingency planning in place for US personnel stationed in Ethiopia, where an ongoing conflict between rebel groups and the central government has continued to deteriorate.

Still, officials emphasized that any potential evacuation in Ukraine likely would not resemble the massive US effort to evacuate all American citizens from Afghanistan earlier this year.

It is not clear how many American civilians are currently in Ukraine -- US citizens are not required to register with the State Department when they relocate abroad.