The Prime Minister and the US President met virtually with the leaders of Nato, the EU, Italy, Poland, France and Germany on Monday evening as they hoped to avert what Mr Johnson warned would be a bloody and protracted conflict for Moscow.

Ukraine Russia© AP Ukraine RussiaDowning Street said they stressed diplomatic discussions with Russia remains the first priority, but said the nation would be hit with “swift retributive responses” if a “further Russian incursion into Ukraine” takes place.

Under efforts to deter Russian “aggression”, Mr Biden said the leaders discussed preparations to “impose severe economic costs” against Moscow while seeking to “reinforce security on the eastern flank”.

The Pentagon put 8,500 US troops on heightened alert, preparing them for a possible deployment to reassure Nato allies amid rising tensions.

Earlier in the day, Mr Johnson warned in an interview that “gloomy” intelligence suggested Russia was planning a lightning raid on Kyiv, as British staff and their families began leaving the Ukrainian capital.

The Prime Minister warned Russian President Vladimir Putin who in 2014 led a Russian annexation of Crimea from Ukraine, that a fresh invasion would be a “disastrous step”.

“The intelligence is very clear that there are 60 Russian battle groups on the borders of Ukraine, the plan for a lightning war that could take out Kyiv is one that everybody can see,” Mr Johnson said.

“We need to make it very clear to the Kremlin, to Russia, that that would be a disastrous step.”

He warned that the people of Ukraine would resist any invasion and “from a Russian perspective, (it) is going to be a painful, violent and bloody business”, he said.

“I think it’s very important that people in Russia understand that this could be a new Chechnya.”

France’s Emmanuel Macron, Germany’s Olaf Scholz, Poland’s Andrzej Duda and Italy’s Mario Draghi also joined the video call on Monday evening, along with European Commission chief Ursula von der Leyen and Nato secretary general Jens Stoltenberg.

“The leaders agreed that, should a further Russian incursion into Ukraine happen, allies must enact swift retributive responses including an unprecedented package of sanctions,” Downing Street said following the discussions lasting over an hour.

Mr Johnson, according to No 10, emphasised the need to support Ukraine’s defences “against the full spectrum of malign Russian activity”.

The United States also ordered the families of all American personnel at the US Embassy to leave the country in response the the risk of an invasion.

But the European Union’s foreign policy chief Josep Borrell said: “We are not going to do the same thing” and Ukraine’s foreign ministry spokesman Oleg Nikolenko said the US decision was “a premature step” and a sign of “excessive caution”.

Downing Street said British combat troops would not be used to defend Ukraine.

The Prime Minister’s official spokesman said around 30 British diplomats, including the ambassador, remained in Ukraine.

An estimated 100,000 Russian troops have massed at the border with Ukraine and intense diplomatic activity has failed to ease tensions.

The Foreign Office said: “Some embassy staff and dependants are being withdrawn from Kyiv in response to the growing threat from Russia.

“The British Embassy remains open and will continue to carry out essential work.”

Meanwhile, Nato committed more ships and fighter jets to eastern Europe.

Denmark is sending a frigate to the Baltic Sea and is set to deploy F-16 fighter jets to Lithuania, Spain is sending ships and is considering sending fighter jets to Bulgaria, France has expressed its readiness to send troops to Romania, and the Netherlands is sending two F-35 fighter planes to Bulgaria from April.

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Russia warned of ‘unprecedented sanctions’ over Ukraine as world leaders talk (msn.com)