US sanctions against Moscow "would hit the United States like a boomerang", Russian foreign minister Sergei Lavrov has warned the US Secretary of State John Kerry.
Mr Lavrov also warned Mr Kerry against "hasty and reckless steps" that could harm relations with Russia during a phone conversation.
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.