The US has ordered the relatives of its embassy staff in Ukraine to leave amid rising tension in the region.
The State Department has also given permission for non-essential staff to leave and urged US citizens in Ukraine to consider departing.
In a statement, it said there were reports that Russia is planning significant military action against Ukraine.
Russia has denied claims that it is planning to invade Ukraine.
The State Department also warned people not to travel to Ukraine and Russia due to the ongoing tension and "potential for harassment against US citizens".
"There are reports Russia is planning significant military action against Ukraine," an advisory from the State Department said.
A State Department official told the AFP news agency that the embassy remains open but repeated warnings from the White House that an invasion could come at "any time".
They said that the government "will not be in a position to evacuate US citizens in such a contingency".
The move by the US is part of a series of precautions the State Department employs when crises could put American diplomats in harm's way, the BBC's Barbara Plett Usher reports.
It is thought that nothing specific over the past 24 hours triggered the decision.
The head of the military defence alliance Nato has warned there is a risk of fresh conflict in Europe after an estimated 100,000 Russian troops amassed on the border.
On Saturday, some 90 tonnes of US "lethal aid" including ammunition for "front-line defenders" arrived in Ukraine.
US Secretary of State Anthony Blinken said the government was putting together a "series of actions that would figure into President Putin's calculus" including beefing up defences in Ukraine with more military assistance.
Russia has seized Ukrainian territory before, when it annexed Crimea in 2014, after the country overthrew its pro-Moscow president.
Ever since, Ukraine's military has been locked in a war with Russian-backed rebels in areas of the east near Russia's borders. An estimated 14,000 people have been killed in the Donbas region.
On Sunday, the UK Foreign Office accused Mr Putin of planning to install a pro-Moscow figure to lead Ukraine's government.
UK ministers have warned that the Russian government will face serious consequences if there is an incursion.
US officials said they are concerned that Russia could try to topple and replace the Ukraine government but declined to comment on the UK foreign office's statement over the weekend.
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