British Prime Minister Boris Johnson and US President Joe Biden agreed in a call on Monday that a "crucial window for diplomacy" remained open for the crisis in Ukraine.
The discussion, the latest by world leaders on the crisis in Ukraine, comes after US intelligence officials gave a warning that an invasion by Moscow into its western neighbour could be days away.
US citizens were told they should leave Belarus immediately due to the build-up of Russian troops along Belarus's border with Ukraine, the State Department said on Monday.
Mr Johnson and Mr Biden "agreed there remained a crucial window for diplomacy and for Russia to step back from its threats towards Ukraine," a British government spokesman said.
"The leaders emphasised that any further incursion into Ukraine would result in a protracted crisis for Russia, with far-reaching damage for both Russia and the world."
They also stressed that diplomatic discussions with Russia remained "the first priority", and welcomed talks that have already taken place between Russia and Nato allies, the spokesman said.
"They agreed that Western allies must remain united in the face of Russian threats, including imposing a significant package of sanctions should Russian aggression escalate," he said.
"They also reiterated the need for European countries to reduce their dependence on Russian gas, a move which, more than any other, would strike at the heart of Russia's strategic interests."
The White House said the two leaders "reviewed ongoing diplomatic and deterrence efforts in response to Russia's continued military build-up on Ukraine's borders and reaffirmed their support for Ukraine's sovereignty and territorial integrity".
"They discussed efforts to reinforce the defensive posture on Nato's eastern flank and underlined the continued close co-ordination among allies and partners, including on readiness to impose severe consequences on Russia should it choose further military escalation," it said.
Russia has massed an estimated 130,000 troops nears its Ukrainian border and in neighbouring Belarus, where they have been involved in joint exercises.
Earlier on Monday, Mr Johnson urged Russian President Vladimir Putin to step back from "the edge of a precipice", calling the situation "very, very dangerous".
The British leader said he had no plans to visit Moscow but that he would be discussing the crisis with "various leaders" soon.
Meanwhile in London, Foreign Secretary Liz Truss later chaired a meeting of the government's emergency Cobra committee, to discuss British citizens who may be stuck in Ukraine in the event of hostilities.
The government is urging all Britons to leave the country by commercial flights, but says it is maintaining a "core" diplomatic presence in Kiev.
"We are doing everything possible to prevent a Russian invasion in Ukraine, while also preparing for the worst," Ms Truss tweeted after the meeting.
Mr Johnson was also to receive a security briefing from intelligence chiefs.
On Tuesday, he will chair a full meeting of the Civil Contingencies Committee, which is convened to handle matters of national emergency or major disruption to discuss the UK's overall response, Downing Street said.
