Downing Street says the crisis on Ukraine's border has reached a "critical juncture" - and Russia could invade "at any moment".
But officials think there is "still a window of opportunity for de-escalation".
UK Prime Minister Boris Johnson's due to hold fresh talks with world leaders before a trip to Europe this week.
And the Guardian's Defence Editor Dan Sabbagh says German Chancellor Olaf Scholz will travel to Moscow on Tuesday.
IN: 'He's supposed...
OUT: ...invade.'
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