In a 40-minute call, Joe Biden and Boris Johnson agreed a deal was still possible despite a chorus of warnings of imminent Russian military action.
Russia has always denied plans to invade Ukraine, despite massing more than 100,000 troops on the border.
On Monday its foreign minister said diplomacy was "far from exhausted".
More than a dozen nations have urged their citizens to leave Ukraine, and the US has said aerial bombardments could begin "at any time".
But in their conversation US President Joe Biden and UK PM Boris Johnson said there remained a "crucial window" for diplomacy, according to a Downing Street statement.
"The Prime Minister and President Biden updated one another on their recent discussions with fellow world leaders," it said.
"They agreed there remained a crucial window for diplomacy and for Russia to step back from its threats towards Ukraine.
"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," the statement added.
Mr Johnson reportedly said that the UK was prepared to do everything it could to help, to which Mr Biden responded: "We're not going anywhere without you pal."
Mr Johnson is expected to hold a Cobra meeting on Tuesday to discuss the UK's response to the pressures.
Photo TASS/Getty Images Caption: Russia says some of its military drills are coming to an end