The US on Monday said it is relocating its Ukrainian embassy from Kiev to the western city of Lviv, citing a "dramatic acceleration" of Russian troops at the country's border.
US officials have warned that Moscow could at any moment attack Ukraine, including Kiev, after amassing more than 100,000 troops close to the eastern border of the former Soviet republic and more forces in Belarus to the north.
US Secretary of State Antony Blinken said in a statement that while the embassy would move to Lviv -- roughly 80 kilometres from Ukraine's western border with Poland -- it "will remain engaged with the Ukrainian government".
President Joe Biden on Monday spoke to British Prime Minister Boris Johnson, as Mr Biden attempts to rally unity against a possible invasion.
The two leaders "agreed there remained a crucial window for diplomacy and for Russia to step back from its threats towards Ukraine," Downing Street said in a readout of the call.
The White House said the pair also "reaffirmed their support for Ukraine’s sovereignty and territorial integrity."
The relocation of the US embassy comes after National Security Adviser Jake Sullivan told Americans in Ukraine to leave immediately.
He suggested the Russian attack could come this week, but noted it was still unclear if Russian President Vladimir Putin had made a final decision to invade.
State Department spokesman Ned Price said Russia had been unable to divide the United States and its allies and partners over the crisis.
"If anything, we are more united as we confront Russia's threats and aggression. We remain committed to keeping the prospect of de-escalation through diplomacy alive," Mr Price said.
The Pentagon said Russia continued to mass its forces on the Ukrainian border over the weekend, despite Moscow announcing it was ending some military drills.
Mr Putin has "well north of 100,000" forces on the border, Pentagon spokesperson John Kirby told CNN.
Earlier, Russian Foreign Minister Sergei Lavrov Sergei Lavrov told Mr Putin there was a "chance" of reaching an agreement on security with the West.
Meanwhile, US Secretary of Defence Lloyd Austin is expected to reiterate the US's commitment to Nato's Article 5 mutual defence clause in a meeting with allied defence ministers and foreign leaders in Belgium on Tuesday.
Agencies contributed to this report.
