The US and its allies will “respond decisively” if Russia invades Ukraine, President Joe Biden has told Vladimir Putin in a phone call.
Mr Biden told his counterpart the US remains prepared to engage in diplomacy despite the build up of Russian troops on the border with Ukraine, but was “equally prepared for other scenarios”.
He added that the US and its allies would “impose swift and severe costs on Russia” in the event of invasion.
The two leaders spoke for more than an hour on the crisis after a build up of Russian troops on the border with Ukraine and intelligence suggesting Moscow could stage an invasion.
It comes after British nationals in Ukraine were warned to leave the country “immediately by any means possible”.
Junior defence minister James Heappey said on Saturday that British troops who are in Ukraine training local forces will depart this weekend and that Britons remaining in the country should not expect military evacuation.
Mr Heappey told Sky News: “British nationals should leave Ukraine immediately by any means possible and they should not expect – as they saw in the summer with Afghanistan – that there would be any possibility of a military evacuation.”
It comes after a similar warning was given to Americans by Mr Biden, as well as from the governments of Ireland, Japan, Latvia, Norway, Canada, Denmark, the Netherlands, Australia, and New Zealand to their citizens in the country.
The US has suspended consular services in the Ukrainian capital of Kyiv and most staff have been told to return home, with a limited embassy presence remaining in Lviv, in the west of the country.
The UK’s ambassador to Ukraine, Melinda Simmons, wrote on Twitter on Saturday: “I am staying in Kyiv and continue to work there with a core team. The embassy remains operational.”
Russia has said staff numbers at its embassy have been “optimised”, with foreign ministry spokeswoman Maria Zakharova saying the move was in response to concerns for their safety.
She said: “We conclude that our American and British colleagues apparently know about some military actions being prepared in Ukraine that could significantly complicate the situation in the security sphere.
“In this situation, fearing possible provocations by the Kyiv regime or third countries, we actually decided to somewhat optimise the staffing of Russian foreign missions in Ukraine.”
According to the Associated Press news agency, US intelligence says a Russian invasion could take place as early as Wednesday.
Earlier on Saturday, US Secretary of State Antony Blinken spoke with Russian Foreign Minister Sergey Lavrov, with the US saying that Mr Blinken “made clear that a diplomatic path to resolving the crisis remained open, but it would require Moscow to deescalate and engage in good-faith discussions.
“He reiterated that should Moscow pursue the path of aggression and further invade Ukraine, it would result in a resolute, massive, and united Transatlantic response.”
Russia has massed troops near the Ukrainian border in recent months, also sending troops to exercises in neighbouring Belarus, despite denying it has plans to invade Ukraine.
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It wants guarantees from the West, including a promise of no missile deployments near its borders, no NATO membership for Ukraine, and a scaling back of the alliance’s military infrastructure.
Although the West has described the Kremlin’s main demands as “non-starters”, it is prepared to discuss steps such as arms control.
In recent days, Russia moved six amphibious assault ships into the Black Sea, augmenting its capability to land marines on the coast.
When asked about the possibility of an invasion within days, Ukrainian president Volodymyr Zelenskyy said on Saturday: “I have to speak with our people, like a president, and say… the truth is that we have different information and now the best friend for enemies that is panic in our country, and all this information that helps only for panic doesn’t help us.”
US troops deployed to Poland
Mr Biden has said the US will not be involved in a war in Ukraine, but it is moving to reassure NATO allies.
The US has ordered an additional 3,000 troops to Poland, and is moving 1,000 soldiers from Germany to Romania which, like Poland, shares a border with Ukraine.
US National Security Adviser Jake Sullivan said: “Russia has all the forces it needs to conduct a major military action.
“Russia could choose, in very short order, to commence a major military action against Ukraine.”
But Ms Zakharova responded by saying: “The hysteria of the White House is more indicative than ever.
“The Anglo-Saxons need a war – at any cost. Provocations, misinformation and threats are a favourite method of solving their own problems.”
In a virtual meeting on Friday evening, UK Prime Minister Boris Johnson spoke with allies and, earlier, Defence Secretary Ben Wallace met Russia’s defence minister Sergei Shoygu.