Any incursion into Ukraine would result in a “protracted crisis” for Moscow with “far-reaching damage for both Russia and the world”, Boris Johnson and Joe Biden have warned.
In a call between the UK prime minister and the US president, the leaders called on Western allies to “remain united”.
But they also said there remained a “crucial window for diplomacy” with Russia.
Mr Johnson has warned that a Russian assault could be launched within 48 hours, describing it as a “very, very dangerous and difficult situation” which has left the world “on the edge of a precipice”.
But he added there was “still time” for Russian President Vladimir Putin to “step back”.
Foreign Secretary Liz Truss earlier chaired a meeting of the government’s emergency COBRA committee to discuss the crisis, with the PM due to lead another such meeting on Tuesday.
“We’re very clear there could be an imminent Russian invasion of Ukraine,” Ms Truss said after Monday’s meeting.
“Our first priority is protecting British citizens – British citizens need to leave now while commercial routes are still available.
“Our focus is on prioritising the safety and security of British nationals in Ukraine.”