US Secretary of State Antony Blinken has said the “imminent” threat of Russian military action in Ukraine justifies evacuating the US embassy in Kyiv.
His words came after Ukraine’s president urged calm, saying the biggest enemy was panic.
More than a dozen countries have urged their citizens to leave Ukraine.
Moscow, with more than 100,000 troops near the border, has denied it plans to invade.
The Kremlin’s top foreign policy advisor Yuri Ushakov has dismissed US warnings of an attack, saying “hysteria has reached its peak”.
Saturday saw further attempts to de-escalate tensions in the region. In a phone call, President Joe Biden warned Russian leader Vladimir Putin of “swift and severe costs” if Russia sends in troops.
UK Defence Secretary Ben Wallace, meanwhile, compared recent Western diplomatic efforts to stop an invasion to the appeasement of Nazi Germany.
Mr Wallace told the Sunday Times newspaper “there’s a whiff of Munich in the air” – a reference to an agreement with Hitler that failed to prevent World War Two.
Mr Wallace also said that an attack remained highly likely and it could come “at any time”.
‘The prudent thing to do’
The UK, US and Germany are among several countries that have urged their nationals to get out of Ukraine immediately.
The US decision to evacuate most of its embassy staff in Kyiv on Saturday was followed by similar moves by Canada and Australia. All three nations have instead shifted operations to the western city of Lviv, near the Polish border – although the UK ambassador has said she will stay in the Ukrainian capital with a core team.
Mr Blinken said the risk of military action “is high enough and the threat is imminent enough” that the evacuation is “the prudent thing to do”.
But earlier Ukrainian President Volodymyr Zelensky had urged calm, saying: “Right now, the people’s biggest enemy is panic.”
Mr Zelensky said that if Western powers had any firm evidence of an impending invasion, he had yet to see it.
The BBC’s Zhanna Bezpiatchuk says there are no major signs of panic in Kyiv or other major Ukrainian cities. However, she added that Ukrainians are starting to take the threat from Russia increasingly seriously and are taking their own contingency measures.
An emergency evacuation plan for Kyiv’s three million residents has been drawn up by the capital’s mayor’s office as a precaution.
The White House has warned that an invasion could happen at any time, and could begin with bombing from the air.
Tensions have steadily increased as Russia has continued to deploy troops along Ukraine’s eastern border. It has also been carrying out its biggest naval drills in years in the Black Sea, with Ukraine accusing the country of blocking its access to the sea.
The crisis comes eight years after Russia annexed Ukraine’s southern Crimea peninsula. Since then, Ukraine’s military has been locked in a war with Russian-backed rebels in eastern areas near Russia’s borders.
The Kremlin says it cannot accept that Ukraine – a former Soviet republic with deep social and cultural ties with Russia – could one day join the Western defence alliance Nato, and has demanded that this be ruled out.
The US has rejected that, saying that as a sovereign nation Ukraine should be free to decide its own security alliances.
