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.
Trump extends deadline for striking Iran's energy plants to April 7
Israeli evacuation orders uproot 370,000 children in Lebanon, UN says
UN rights chief urges US to conclude probe into deadly Iran school strike
No injuries reported as Kuwait Shuwaikh Port comes under drone attack
IRGC-linked cells targeting Bahrain referred to Criminal Court
Austria plans social media ban for children under 14
Trump says Iran's 'present' to US was allowing 10 oil tankers through Hormuz
EU agrees to fine online platforms importing unsafe products
