US Secretary of State John Kerry said there are "contingencies" in place in case Russian forces move farther east into Ukraine.
Although Kerry stressed he hopes to avoid that happening, he said there was "no telling that we can".
Barack Obama warned Russia that the West will be forced to apply a cost to Moscow if it fails to change course in its dispute with Ukraine.
The ballot paper that the people of Crimea will use in Sunday's referendum was published today
Vitali Klitschko's visit to the eastern city of Donetsk is clearly an attempt to try to ease the divisions with Ukraine.