A top White House aide says it's possible that Russia could invade eastern Ukraine, and even U.S. military assistance would be unlikely to prevent it.
Deputy national security adviser Tony Blinken says Russia seems to be trying to intimidate Ukrainians by massing thousands of troops along the border.
But Blinken also tells CNN's "State of the Union" that "it's possible they are preparing to move in."
He says the U.S. is looking at providing military assistance to Ukraine. But he also says "it's very unlikely to change Russia's calculus and prevent an invasion."
Russia's defense chief has told Defense Secretary Chuck Hagel that Russia had no intention of crossing into Ukrainian territory.
Blinken says economic penalties are working to isolate Russia.