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.

After meeting Nato secretary-general Jens Stoltenberg in Brussels, 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 also 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”.

In the UK, business minister Paul Scully warned that Moscow had amassed 7,000 extra troops on the Ukrainian border within the past few days.

“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,” he told Sky News.

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.

Boris Johnson signalled that the prospect of Mr Putin still being “willing to engage in finding a diplomatic solution” was a “welcome sign”.

Ukrainian army soldiers
Ukrainian Army soldiers (AP)

But No 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”.

White House press secretary Jen Psaki said “we are committed to pursuing diplomacy until the moment an invasion begins”, but noted that “currently, Russia appears to be continuing preparations for a full-scale assault on Ukraine very soon”.

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.

“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,” he told TalkRadio.

Defence Secretary Ben Wallace will take questions in the Commons on Monday afternoon.

Russia and ally Belarus said they were extending war games on Belarusian territory that would offer a convenient staging post on Ukrainian capital Kyiv, less than 50 miles south of the border with Belarus.

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.