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.
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A bomb shelter underneath Kiev's Central Train Station. The underground shelter can offer protection to the workers on shift and about 2,000 civilians. AP -
An aerial image of attack helicopters on the runway at Zyabrovka airfield in Belarus. AP -
A Ukrainian servicewoman disinfects the wounds of a dog injured by razor wire at a frontline position outside Popasna, the Luhansk region, in eastern Ukraine. AP -
An interior view shows a kindergarten, which according to Ukraine's military officials was damaged by shelling, in Stanytsia Luhanska in the Luhansk region, Ukraine. Reuters -
Ukrainian pensioner Lidiya Silina, 87, at her house right next to the border with Belarus. Ms Silina would take the rubbish out on the Belarusian side of the border opposite the twisty stream. Then the troubles started and the Russian tanks came. AFP -
A Ukrainian serviceman at a frontline position outside Popasna in eastern Ukraine. AP -
A child copies the position of Ukrainian servicemen standing at attention during the national anthem in Sievierodonetsk, eastern Ukraine. AP -
Ukrainian soldiers pose for a picture as the country observes a Day of Unity in Odessa. AP -
Lorries queue at the Goptivka border crossing between Ukraine and Russia, near the Eastern Ukrainian city of Kharkiv, amid growing tension. EPA -
Ukrainian border guards patrol along the border with Russia, near Kharkiv. EPA -
Four Royal Air Force Typhoon jets in Cyprus. The UK sent the additional jets to boost air patrols over eastern Europe. PA -
Ukrainian President Volodymyr Zelenskiy, second from right, is briefed on preparations at the site of a military drill outside the city of Rivne. AFP -
Nato defence ministers in Brussels, Belgium. The military alliance says Russia is still sending troops to the border with Ukraine, contradicting Moscow’s claims of a drawdown. Reuters -
Ukraine's Senkivka checkpoint near the border with Belarus and Russia. Reuters -
The Motherland Monument in Kiev, Ukraine. The country is on high alert for the possibility of an invasion by its neighbour Russia. AP -
Valentyna Konstantynovska, 79, holds a weapon during basic combat training for civilians in Mariupol, eastern Ukraine. AP -
A satellite image shows a housing area and equipment for troops in Rechitsa, Belarus, where Russian forces are holding joint military exercises. Reuters -
US soldiers prepare to leave Fort Bragg, North Carolina, for Europe, where they are being sent as the crisis between Russia and Ukraine escalates. AFP -
Ukrainian soldiers at the site where a rocket landed close to their positions during the night on a front line near Popasna, eastern Ukraine. AP -
Ukrainian servicemen survey an area that was shelled overnight on Tuesday close to their positions on a front line outside Popasna, eastern Ukraine. AP -
A Ukrainian teenager helps a church minister reinforce basements in a centre for children in the Ukrainian village of Chervone. AFP -
A Ukrainian teenager digs trenches for soldiers serving on their country's eastern front and facing off with Russian-backed separatists, near Chervone. AFP -
A Ukrainian border guard at a crossing to Belarus in Vilcha. Getty -
A child tries to remove ammunition from a clip during basic combat training for civilians organised by the National Guard in eastern Ukraine. AP -
Rumia, 59, a member of Ukraine's Territorial Defence Force, trains near the capital Kiev. AP -
Smoke rises over the Gozhsky training ground during Russia-Belarus military drills in Belarus. AP -
Ukrainians march to show solidarity and patriotic spirit in Kiev. Getty -
A Ukrainian National Guard member shows civilians in the Donetsk region, eastern Ukraine, how to adopt a firing position. AP -
A military helicopter fires missiles during the Russia-Belarus military drills in Belarus. AP -
A pedestrian passes through the only open crossing point between territory held by pro-Russian separatists and Ukrainian government controlled territory in Stanytsia Luhanska, eastern Ukraine. AP -
US soldiers on parade during the visit of Nato Secretary General Jens Stoltenberg to the Mihail Kogalniceanu airbase, near Constanta, eastern Romania. AP
