The American Secretary of State, Antony Blinken, has said that the "immediate" threat of Russian military action in Ukraine justifies the evacuation of the American embassy in Kiev.
His words came as Ukraine's president called for calm, saying the biggest enemy was panic.
Dozens of countries have asked their citizens to leave Ukraine.
Moscow, with more than 100,000 troops near the border, has denied plans to invade. The Kremlin's top foreign policy adviser, Yuri Ushakov, dismissed US warnings of an attack, saying "hysteria has reached its peak".
Saturday saw further efforts to reduce tensions in the region. In a phone call, President Joe Biden warned Russian leader Vladimir Putin of "swift and heavy costs" if Russia sends troops, reports BBC.
The United Kingdom, the United States and Germany are some of the countries that have asked their citizens to leave Ukraine immediately.
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US closes embassy in Kiev for fear of major Russian attack
The US decision to evacuate most of its embassy staff in Kiev on Saturday followed similar moves by Canada and Australia.
All three countries have moved operations to the western city of Lviv, near the Polish border, although the UK ambassador has said she will remain in the Ukrainian capital with a core team.
Blinken said that the risk of military action "is quite high and the threat is quite imminent", so he emphasized that evacuation is a prudent step to take.
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