Prime Minister Boris Johnson has warned Russia that invading Ukraine would be “disastrous” and a “painful, violent and bloody business”.
Speaking as the Foreign Office pulled some embassy staff out of Ukraine, the PM said the situation is “pretty gloomy” but war was not inevitable.
He said the UK was “leading on creating a package of economic sanctions” against Russia.
Defensive weaponry is being supplied to Ukraine, Mr Johnson said.
He added that the UK stood “four-square with the people of Ukraine”.
Russia has denied plans for military action, but tens of thousands of troops have amassed on the border.
Mr Johnson said: “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.
“We need to make it very clear to the Kremlin, to Russia, that that would be a disastrous step.”
Officials say there have been no specific threats to British diplomats but about half of the staff working in Kyiv will return to the UK. The US has ordered relatives of its embassy staff to leave, saying an invasion could come “at any time”.
The embassy moves were described as precautionary, and nothing specific is thought to have occurred in the past 24 hours to have triggered the decisions of the US and UK.
EU staff in Ukraine will stay in place for now, with EU foreign policy chief Josep Borrell saying he would not “dramatize” the tensions.
Members of the Nato alliance, including Denmark, Spain, Bulgaria and the Netherlands, are sending more fighter jets and warships to Eastern Europe to bolster defences in the region. With an estimated 100,000 Russian troops now at the border with Ukraine, the head of Nato has warned there is a risk of fresh conflict in Europe.
The prime minister said he had visited Ukraine and knew the people of the country, adding: “My judgment is that they will fight.”
Mr Johnson, speaking to reporters in Milton Keynes, said: “We also need to get a message that invading Ukraine, from a Russian perspective, is going to be a painful, violent and bloody business.
“I think it’s very important that people in Russia understand that this could be a new Chechnya.”
There was a major separatist conflict in Chechnya in the 1990s, with a decade of ultimately unsuccessful fighting for its independence.
Asked if an invasion was imminent, Mr Johnson said: “I’ve got to tell you that I think the intelligence is pretty gloomy at this point. There is certainly a very, very large array of Russian forces and we have to take the necessary steps.
“I don’t think it’s by any means inevitable now, I think that sense can still prevail.”
He is due to speak to international allies on Monday.
Russia has seized Ukrainian territory before, when it annexed Crimea in 2014, following fierce protests in Ukraine that toppled the country’s pro-Russian president. Russian forces seized control of Crimea before the territory voted to join Russia in a referendum the West and Ukraine deemed illegal.
Russian-backed rebels control areas of eastern Ukraine near Russia’s borders in a conflict which has cost an estimated 14,000 lives. A 2015 peace deal is far from being fulfilled.
On Sunday, the UK Foreign Office accused Mr Putin of planning to install a pro-Moscow figure to lead Ukraine’s government.
The man named by the UK Foreign Office – former Ukrainian MP Yevhen Murayev – called the claims “stupid” in an interview with Reuters news agency.
The Foreign Office has warned against all travel to Crimea and two areas of eastern Ukraine, Donetsk oblast and Luhansk oblast. It is advising against all but essential travel to the rest of Ukraine.