James Heappey said the small number of UK personnel sent to train Ukrainian troops on the anti-tank missiles “will be withdrawn,” alongside around 100 helping with wider troop training on Operation Orbital.Photograph: Vyacheslav Madiyevskyy/Reuters© Provided by The Guardian Photograph: Vyacheslav Madiyevskyy/ReutersBritain, in common with all other Nato allies, has said it would not fight against any Russian attack, a point repeated by the minister in a BBC interview. Ukraine is not a member of Nato, although in 2008 it was given a promise that it would one day be able to join.

“There will be no British troops in Ukraine if there is to be a conflict there,” Heappey said: “They will be leaving over the course of the weekend.”

The military evacuation follows a string of warnings, led by the US but supported by the UK, that Russia has assembled a force capable of invading Ukraine. Jake Sullivan, the US national security adviser, said on Friday night: “We are in the window when an invasion could begin at any time.”

Western intelligence fear the most likely scenario, in the event of a conflict, is that Russian forces would mount a lightning attack aimed at encircling Kyiv, a city of more than 3 million people, and trying to force a change of regime. As a first step, Moscow’s forces would aim to rapidly degrade Ukraine’s military in a lightning attack.

Russia was now in a position to attack “very, very quickly”, Heappey said, though he hoped no invasion would take place. He told BBC Breakfast: “We are now confident that the artillery systems, the missile systems and the combat air are all in place that would allow Russia to launch – at no notice – an attack on Ukraine.”

The military withdrawal follows a Foreign Office warning on Friday to British nationals, telling them to leave Ukraine immediately while commercial flights are still available and the land border with Poland remains open.

The US told all remaining Americans to leave the country in the next 48 hours, on Friday, with a string of countries following suit with similar warnings. Australia, New Zealand, Japan, Germany, Latvia, Norway and the Netherlands are among those issuing similar advice. Israel said it was evacuating relatives of embassy staff

There are no plans for an emergency airlift by western air forces from Kyiv, unlike in Afghanistan last summer, the minister said. Unlike then, “the Royal Air Force will not be in a position to go in and to fly people out”, Heappey added.

But US and UK forces are building up in Poland to help provide support to the eastern European country and help out if there is a mass movement of refugees in the event of an invasion. On Friday, the US said it would send 3,000 further troops to Poland, on top of 1,700 it had already agreed to send out.

Britain said earlier this week it would send 350 troops to Poland, in addition to the 250 it already has in the country, and has placed a further 1,000 on standby for deployment to help with any refugee movements. The US has estimated that anywhere between 1m and 5m people could be displaced.

Russia has repeatedly denied it has any plans to invade and its defence minister criticised Britain on Friday for delivering anti-tank weapons to Ukraine and helping with military training.

Sergei Shoigu, in a meeting with his UK counterpart Ben Wallace, also accused the UK of sending special forces to the country. “I would like to see the reason why the United Kingdom is sending special forces to Ukraine and until when [they] will be there,” Shoigu said.

On Saturday, the US president Joe Biden will speak with Vladimir Putin by phone in an attempt to resolve the crisis.

Amid calls for Britons to leave the country as quickly as possible, the former British ambassador to the US Lord Kim Darroch described the challenge remaining embassy staff will have to undertake to help Britons leave the country, saying it will “overwhelm the embassy’s resources.”

. “With lots of foreign nationals trying to get out, there will be chaos, there may need to be extra flights laid on so this will occupy everyone’s time for 24 hours a day for the next few days and you won’t get everyone out – some people will choose to stay,” he said.