Foreign Secretary Liz Truss has warned that a Russian invasion of Ukraine appears "highly likely" despite Joe Biden and Vladimir Putin tentatively agreeing to hold a crisis summit.
The US president agreed during last-ditch diplomatic efforts, against the backdrop of heightening tensions, to meet his Russian counterpart on the condition Moscow does not invade.
Ms Truss, however, did not appear to be revising her concerns that the Kremlin would order an attack as she warned that the price of an invasion must be "intolerably high" for Russia.
Ms Truss said the UK and allies are "stepping up preparations for the worst-case scenario" after a meeting today with NATO Secretary General Jens Stoltenberg.
She tweeted: "Diplomacy must be pursued but a Russian invasion of Ukraine looks highly likely.
"The UK and allies are stepping up preparations for the worst-case scenario. We must make the cost for Russia intolerably high."
The prospect of talks did little to dampen fears an attack was imminent in Washington, with the White House saying the Kremlin was continuing to prepare a "full-scale assault on Ukraine very soon".
UK Business Minister Paul Scully warned that Moscow had amassed 7,000 extra troops on the Ukrainian border within the past few days in line with the previously reported extension of Russian military drills near the Ukrainian border.
Talking to Sky News, Mr Scully said: "So there is a very, very credible threat and that's why we've got to continue to be vigilant, we've got to continue to work with Ukraine and Poland, as Liz Truss, the Foreign Secretary, was doing just this week."
The minister also warned "the loss of life will be horrendous" if Mr Putin does not engage in diplomacy after French President Emmanuel Macron sought to broker a meeting during a series of calls.
Watch: PM Boris Johnson speaking at the Munich Security Conference.
Prime Minister Boris Johnson, who previously said that a Russian invasion of Ukraine could lead to the largest conflict since WW2, has also signalled that the prospect of Mr Putin still being "willing to engage in finding a diplomatic solution" was a "welcome sign".
However, Number 10's account of the Prime Minister's own call with Mr Macron during the diplomatic flurry over the weekend did not appear overly optimistic about the prospect of a Russian climbdown.
The leaders "underscored the need for President Putin to step back from his current threats and withdraw troops from Ukraine's border", Downing Street added.
Mr Macron's office said the Russian and US presidents had both "accepted the principle" of a summit, adding that the meetings "can only be held on the condition that Russia does not invade Ukraine".
Watch: What are false flag attacks and what would one look like?
Business minister Mr Scully sought to deny suggestions Mr Macron had been more effective in tackling the Ukraine crisis than the Prime Minister, saying Mr Johnson had been on the "front foot" in addressing the rising tensions.
Speaking to TalkRadio he said: "It's important that every member of NATO collaborate and work together, as they are, to make sure that Vladimir Putin realises that he needs to go down the diplomatic route and retreat from the border".
Heavy shelling in Ukraine continued on Monday in the heightened tension between Ukrainian forces and Russian-backed separatist rebels in the Donbas region.
Mr Johnson and other Western allies have suggested the shelling was part of a "false flag" attempt by the Russians to stage a pretext to attack.
Russia has always denied it is planning to invade Ukraine.