Boris Johnson and Joe Biden have warned an invasion of Ukraine would result in a “protracted crisis” for Russia.
During a call on Monday evening, the two leaders also agreed, however, that there remained a “crucial window” for diplomatic talks, after the prime minister urged the Russian president to step back from the “edge of a precipice”.
“The leaders emphasized that any further incursion into Ukraine would result in a protracted crisis for Russia, with far reaching damage for both Russia and the world,” a No 10 spokesperson said.
They added Mr Johnson and the US president “agreed that western allies must remain united in the face of Russian threats, including imposing a significant package of sanctions should Russian aggression escalate”.
The remarks follow the prime minister’s warning that a Russian invasion of the Eastern European country could come within the “next 48 hours”.
Mr Johnson, who cut short an official visit to Cumbria in order to return to No 10, received a security briefing from intelligence chiefs on Monday and will chair an emergency Cobra meeting on Tuesday as the crisis threatens stability in Europe.
Ahead of his call with the US president, the prime minister stressed the evidence was “pretty clear” that the Kremlin was planning for an invasion, with an estimated 130,000 Russian troops massing on the Ukrainian border.
“This is a very, very dangerous, difficult situation, we are on the edge of a precipice but there is still time for President [Vladimir] Putin to step back,” he insisted.
Mr Johnson called for more dialogue and urged Russia to avoid a “disastrous” invasion, and said Mr Putin needed to understand the economic and political consequences of an incursion.
Asked whether an invasion could be hours or days away, he replied: “The signs are from president [Joe] Biden they are at least planning for something that could take place as early as the next 48 hours. That is extremely concerning.”
Earlier, Liz Truss also agreed to a Cobra meeting in Whitehall to discuss the “consular response to the crisis”, after British nationals were advised to leave Ukraine on Friday, while commercial flights remain an option.
The foreign secretary, who had a frosty encounter with her Russian counterpart Sergei Lavrov in Moscow last week, echoed Mr Johnson’s comments, telling reporters Mr Putin could launch an invasion of Ukraine “almost immediately”.
A government source told The Independent: “This is part of our effort to make sure we’re fully prepared. Liz wants to leave nothing to chance.
“Her and the PM are leading the last minute diplomatic charge across Europe this week, while at the same time doing everything possible to prepare for an invasion.”
Elsewhere, Ukraine’s ambassador to the UK, Vadym Prystaiko, backtracked on suggestions the country could consider dropping its constitutional ambition to join Nato to avoid war.
The country’s foreign minister, Dmytro Kuleba, added: “Ukraine’s strategic course on joining Nato remains unchanged. It is enshrined in our constitution and national foreign policy strategy, supported by a growing majority of Ukrainians. It’s only up to Ukraine and thirty Nato allies to decide on the issue of membership”.