Boris Johnson will chair a meeting of the Cobra emergency committee to consider the latest developments in the Ukraine crisis as hopes rise that diplomacy may still avert war.

The Prime Minister held talks with US President Joe Biden on Monday where they agreed a “crucial window for diplomacy” remained open, while Russian President Vladimir Putin and his foreign policy chief Sergei Lavrov have also agreed to continue down the path of negotiations.

A Downing Street source said there was “some hope” that diplomacy could work and Russia could step back.

Russia’s defence ministry said some of its troops in the regions bordering Ukraine were returning to base following the completion of military exercises.

But Foreign Secretary Liz Truss stressed the threat of an invasion remained as she confirmed some British embassy staff had moved out of the capital Kyiv to Lviv in western Ukraine, further away from the troops massing on the Russian side of the border.

The US has closed its Kyiv embassy and moved remaining staff to its mission in Lviv, but Ms Truss said the UK would retain a presence in the capital.

“We have moved some staff to Lviv in the west of Ukraine. We still have staff in Kyiv,” she told BBC Radio 4’s Today.

“We continue to review that. But we do think it is very important that we are supporting British citizens in Ukraine and we do have a presence in Kyiv, but, clearly, I’m constantly looking at our staff safety to make sure they are protected.”

She warned that a Russian invasion could reach the capital “very, very quickly” – and suggested a “false flag” operation might be mounted within days so Moscow could claim justification for an attack.

Russian tanks
Russian tanks roll on the field during a military drills in Leningrad (Russian Defence Ministry Press Service via AP)

“They have troops stationed around Ukraine. So, there could be an attack on Kyiv. There could be an attack from the east,” she told Sky News.

“What we do expect over the next few days is there could be an attempt of a false flag operation to create a pretext to claim the Ukrainians are attacking them, so that the Russians have a justification for invading Ukraine.”

Reports from Russia indicated that units of the southern and western military districts were returning to base following the conclusion of training exercises.

The news broke while Ms Truss was conducting a round of broadcast interviews and she told LBC Radio she had not seen the reports and “would need to see more details to understand if that has any major implications”.

The Government was keen to stress the diplomatic process is continuing.

“There is some hope on there still being a window for diplomacy to work and Russia stepping back,” a Downing Street source said.

Ms Truss said “we believe that Vladimir Putin has not yet made a decision about whether to invade Ukraine” and “we know that they’re in a position to attack imminently, but he can still change his mind and that is why diplomacy is so vital”.

One possible path out of the crisis could come in the form of reassurances to Russia about the timeline for Ukraine’s future Nato membership – one of the concerns highlighted by Mr Putin’s administration.

Ukrainian President Volodymyr Zelensky said on Monday that Nato membership was “like a dream” but “we don’t know when it will happen”.

German Chancellor Olaf Scholz, who met Mr Zelensky on Monday and was holding talks with Mr Putin on Tuesday, said Ukraine joining the Nato alliance was “practically not on the agenda”.

Ukraine leader
Ukrainian President Volodymyr Zelensky (AP)

The Prime Minister cut short a planned overnight stay in Cumbria on Monday, instead returning to Downing Street to chair Tuesday’s Cobra meeting.

He received a briefing on Monday from the UK’s intelligence chiefs, who presented the latest information on the Russian military build-up.

The UK and Western allies have warned that any Russian incursion into Ukraine would be met with an unprecedented package of sanctions.

That could include shutting down the Nord Stream 2 pipeline intended to carry gas from Russia to Germany, although Mr Scholz has been reluctant to explicitly commit to that despite Mr Biden’s insistence it could not go ahead if troops invaded Ukraine.

Ms Truss told BBC Breakfast that a war in Ukraine would have a “very damaging impact” on oil and gas prices across Europe.

“This is why we need to reduce dependence on Russian gas, find alternative sources,” she said.