The US Embassy in Ukraine's capital Kyiv was temporarily closed on Wednesday, as the mission received information of a "potential significant air attack."
The move comes a day after Ukraine launched US long-range missiles into Russian territory for the first time on Tuesday, using ATACMS missiles against a facility in the Bryansk region close to the border.
Russia warned on Tuesday that it would respond.
A statement released by the Embassy advised employees to shelter in place and "recommends US citizens be prepared to immediately shelter in the event an air alert is announced."
Further details regarding the potential attack have not been disclosed.
Russia had been warning the West for months that if Washington allowed Ukraine to fire US, British and French missiles deep into Russia, Moscow would consider those NATO members to be directly involved in the war in Ukraine.
On Tuesday, Putin lowered the threshold for a nuclear strike in response to a broader range of conventional attacks, with nuclear risks rising amid the highest tensions between Russia and the West in more than half a century.
One of Afghanistan's worst earthquakes killed more than 800 people and injured at least 2,800, authorities said on Monday, as helicopters ferried the wounded to hospital after they were plucked from the rubble of homes being combed for survivors.
Israel pushed tanks deeper into Gaza City and detonated explosives-laden vehicles in one suburb as airstrikes killed at least 19 people on Monday, Palestinian officials and witnesses said.
India's Narendra Modi told Vladimir Putin on Monday that India and Russia stood shoulder to shoulder even in difficult times after the Kremlin chief cast the Indian prime minister as his "dear friend" and gave him a lift in his armoured limousine.
Chinese President Xi Jinping urged leaders to leverage their "mega-scale market", while Russian President Vladimir Putin showed support for Xi's ambition for a new global security and economic order that poses a challenge to the US during a regional summit on Monday.
South Korea has suspended a military radio broadcast that transmits to North Korea as part of measures aimed at easing tensions with Pyongyang, Seoul's defence ministry said on Monday.