The Guardian can confirm that the warning of an imminent attack has gone out to Ukraine and allies and that the UK shared the US assessment.
Diplomats I have been talking to are aware the West has cried wolf once before, on 16 February, so there are caveats.
There was also a scare last night, when some intelligence suggested an all-out attack was about to happen but it turned out to be a false alarm.
The bottom line is there are large Russian formations on the move, so the assessment is that an onslaught could be unleashed at any time.