British Prime Minister Boris Johnson warned Russian President Vladimir Putin on Monday that an invasion of Ukraine would be a "disastrous step."
"We need to make it very clear to the Kremlin, to Russia, that that would be a disastrous step," Johnson told broadcasters.
An invasion would be a "painful, violent and bloody business," he said.
Johnson said on Monday the intelligence around a Russian invasion of Ukraine was gloomy but not inevitable, as he warned President Vladimir Putin that any conflict could become "a new Chechnya".
Asked whether he thought an invasion was now imminent, he said: "The intelligence is pretty gloomy on this point.
"I don't think it's by any means inevitable now, I think that sense can still prevail."
Johnson said that he would be speaking with fellow leaders on the matter later on Monday.
"We also need to get over the message that invading Ukraine, from a Russian perspective, is going to be a painful, violent and bloody business and I think it's very important that people in Russia understand that this could be a new Chechnya."
NATO said on Monday it was putting forces on standby and reinforcing eastern Europe with more ships and fighter jets, in what Russia denounced as an escalation of tensions over Ukraine.
The move added to a flurry of signals that the West is bracing for an aggressive Russian move against Ukraine. The Kremlin, in response, accused the West of "hysteria".
