The warning came as 65,000 soldiers appeared to be at battle stations, heightening fears they were about to launch assaults on cities including Odessa and the capital Kiev.
Tensions have reached boiling point with US President Barack Obama saying America and Europe must join forces to impose further sanctions on Russia.
He said that “we’re going to be in a stronger position” when Russian president Vladimir Putin “sees that the world is unified rather than this is just a US-Russian conflict”.