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 was "pretty gloomy" but war was not inevitable.
He said the UK was "leading on creating a package of economic sanctions" against Russia and was supplying defensive weaponry to Ukraine.
Nato is putting forces on standby.
It said Nato allies were sending additional ships and fighter jets to eastern Europe to reinforce defences and increase deterrence, in response to the continuing build-up of Russian forces.
Russia has denied plans for military action, but an estimated 100,000 troops have amassed on the border. The head of Nato has warned of a risk of fresh conflict in Europe.
The Kremlin has accused Nato of escalating tensions with the move. Russia considers the alliance as a threat, and is demanding legal guarantees that it will not expand further east, including into Ukraine. But the US has said the issue at stake is Russian aggression, not Nato expansion.
US President Joe Biden is due to hold a video conference with EU leaders, the Nato secretary general and Mr Johnson later.
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."
Mr Johnson added that the UK stood "four-square with the people of Ukraine".
