It comes as Defense Secretary Ben Wallace warned Russia could mount an invasion “at any time”. He warned that conflict would have “tragic consequences” for both countries.
Following talks in Moscow with his Russian counterpart Sergei Shoigu, Mr Wallace said he had received an assurance the Kremlin was not planning to attack its southern neighbour.
But with 130,000 Russian troops massed along the borders and large-scale military exercises taking place in Belarus, he said they would judge such assurances by Moscow’s actions.
A Foreign Office spokesman said on Friday evening: “The safety and security of British nationals is our top priority, which is why we have updated our travel advice.
“We urge British nationals in Ukraine to leave now via commercial means while they remain available.”
His news took place as Boris Johnson joined other Western leaders on a conference call organized by US President Joe Biden to discuss the situation in the region.
Also taking part were French President Emmanuel Macron, German Chancellor Olaf Scholz and Nato secretary-general Jens Stoltenberg, as well as EU leaders Ursula von der Leyen and Charles Michel.
Following a frosty meeting in Moscow on Thursday between Foreign Secretary Liz Truss and her counterpart Sergei Lavrov, Mr Wallace said his discussions with Mr Shoigui had been “frank and constructive”.
While he said that he took the minister’s assurances “seriously”, he admitted that he was less optimistic than he had been previously that there could be a diplomatic solution to the crisis.
He said the current disposition of Russian forces meant they could do “a whole range of actions, including an invasion of a neighboring country, at any time”.
“I was clear about the tragic consequences that any invasion of Ukraine could have for all people – both Ukrainian (and) Russian – and the security of Europe,” he said.
“I think we have had a constructive and frank discussion and I hope it has contributed to a better atmosphere but also to de-escalation, but there is still considerable way to go between the two of us.”
He said they had he discussed “confidence building measures” as well as the importance of implementing the Minsk Agreement, brokered by France and Germany, which was supposed to end the fighting between the Ukrainian military and Russian-backed separatist rebels in the east of the country.
“I reiterated that, to its very core, Nato is defensive. We are not interested in dividing and ruling Russia. We are not seeking confrontation,” he said.
He said that 2,000 anti-tank missile launchers supplied by the UK to the Ukrainian military were purely defensive.
I have added: “They’re not strategic, they’re short range. They are designed really for the protection of infantry at short range from armor he said.
“They in no way would pose a threat to an external state as long as that state did not invade that country.”
Mr Wallace denied reports the UK was planning to send 600 Special Forces troops to Ukraine, saying it had only a small number of military trainers in the country whose presence had been fully declared.