Britain on Tuesday said "there is still time" for Russian President Vladimir Putin to step back and recall troops massed on Ukraine's border.
"We could be on the brink of a war in Europe, which would have severe consequences not just for the people of Russia and Ukraine, but also for the broader security of Europe," Foreign Minister Liz Truss told Sky News.
"There is still time for Vladimir Putin to step away from the brink. But there is only a limited amount of time for him to do that," she added, warning an invasion could be "imminent".
Truss held frosty talks last week with Russian counterpart Sergei Lavrov in Moscow. He called it "a conversation between a mute person and a deaf person."
Prime Minister Boris Johnson on Monday urged Putin to step back from "the edge of a precipice", warning that an invasion of Ukraine could come within 48 hours.
"You've got about 130,000 troops massing on the Ukrainian border. This is a very, very dangerous, difficult situation," Johnson told reporters on a visit to Scotland.
"We are on the edge of a precipice but there is still time for President Putin to step back," he added.
Johnson will on Tuesday chair a meeting of the Civil Contingencies Committee that is convened to handle matters of national emergency or major disruption to discuss the UK's response to the crisis.
The government on Friday urged all Britons to leave Ukraine country by commercial flights, but says it is maintaining a "core" diplomatic presence in Kyiv.
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