Boris Johnson has said an attack on a kindergarten in Ukraine was a “false flag operation designed to discredit the Ukrainians”.
“We fear very much that that is a thing we will see more of over the next few days,” the prime minister said on Thursday afternoon.
Western allies have suggested Russia is attempting to manufacture a pretext to invade.
Ukraine has accused Russia-backed separatists in Donbas, in the east of the country, of shelling a village which led to a kindergarten being hit.
US President Joe Biden has said that there is a “very high” risk of a Russian invasion and that could happen within “several days”.
Russia released footage in recent days claiming to show troops and equipment moving back to base following military exercises in regions close to the Ukraine border.
But the British government and other Western leaders have said they have not seen any deescalation on the ground.
Johnson said there was “still time for the Putin regime to step back” from war.
“There is still time to avoid a catastrophe, a catastrophe for Russia, a catastrophe for Ukraine and for the world,” he said.
“If Russia were so mad as to invade, I don’t think people should imagine that this would be a brief business.
“This would be a bloody and protracted conflict in which, I’m afraid, there will be many casualties and including many Russian casualties. I just hope that people in Russia can see that.”
Earlier Nato secretary-general Jens Stoltenberg said: “We are concerned that Russia is trying to stage a pretext for an armed attack against Ukraine.
“There is still no clarity, no certainty about Russia’s intention.”
US defence secretary Lloyd Austin told a press conference that far from drawing down at the border, Russia was adding to the “more than 150,000 troops”.
The UK has ruled out sending troops into Ukraine to counter any Russian invasion, as Ukraine is not a Nato member.
But British forces have been committed to efforts to bolster defences on Nato’s eastern flank.
The UK has already said it is doubling its number of troops in Estonia, with around 850 members of the Royal Welsh battlegroup heading from Sennelager in Germany and bases in Europe for the Baltic state, while 350 Royal Marines are being despatched to Poland.