The UK has begun to withdraw staff from the British embassy in Ukraine amid warnings of a Russian invasion.
Officials said there has been no specific threats to British diplomats, but about half of the staff working in Kyiv will return to the UK, reports BBC.
It comes as the US ordered the relatives of its embassy staff to leave Ukraine, saying a Russian invasion could come "at any time".
Russia has denied having any plans for military action in Ukraine.
The US 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.
However the move is a precautionary one, and nothing specific is thought to have occurred in the past 24 hours to have triggered the decision.
Non-essential US embassy staff have also been told they can leave, and US citizens have been urged to do the same.
The US has stressed that it is not an evacuation, but the State Department told the AFP news agency that if there was a Russian invasion, it "will not be in a position to evacuate US citizens".
The European Union will not be doing the same at the moment. EU Security Policy chief Josep Borrell said he will not "dramatise" the tensions.
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, following fierce protests in Ukraine that toppled the country's pro-Russian president. Russian forces seized control of Crimea before the territory voted to join Russia in a referendum the West and Ukraine deemed illegal.
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.
The man named by the UK Foreign Office - former Ukrainian MP Yevhen Murayev - called the claims "stupid" in an interview with Reuters.
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.