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”.
Many European countries are wary of straining their business ties with Moscow.
British Foreign Secretary William Hague said that there would be “an expansion of existing sanctions in the coming days”.
Russia Plans To Invade Ukraine