"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, echoing US warnings that an invasion could be imminent.

Johnson called on Western allies to "stand together and show a united front", and on European leaders to learn the lessons from Moscow's annexation of Crimea in 2014, and reduce their dependence on Russian gas.

"All European countries need to get (gas pipeline) Nord Stream out of the bloodstream, yank out that hypodermic drip-feed of Russian hydrocarbons that is keeping so many European economies going," he said.

Johnson said he had no plans to visit Moscow, but that he would be discussing the crisis with "various leaders, including (US President) Joe Biden, very soon."

In London, Foreign Secretary Liz Truss later chaired a meeting of the government's emergency "COBR" committee, to discuss British nationals who may end up stuck in Ukraine in the event of hostilities.

The government is urging all Britons to leave the country by commercial flights but says it is maintaining a "core" diplomatic presence in Kyiv.

"We are doing everything possible to prevent a Russian invasion in Ukraine, while also preparing for the worst", Truss tweeted after the meeting.

The prime minister was also to receive a security briefing from intelligence chiefs, and will on Tuesday chair a full meeting of the Civil Contingencies Committee that is convened to handle matters of national emergency or major disruption to discuss the UK's overall response, Downing Street said.