The US Embassy in Kyiv said it has received warning of a potentially significant Russian air attack on Wednesday and would be closed as a precaution.
In a statement, the Embassy also instructed employees to shelter in place and also recommended that US citizens in Kyiv be prepared to immediately shelter in the event of an air alert.
The warning was unusual for its specificity as Russian air attacks have become a common, near-daily occurrence in Ukraine.
But it comes one day after Moscow said US-made longer-range missiles had been used in a Ukrainian attack that struck a weapons warehouse in the Bryansk region after US President Joe Biden authorised their use.
Russian President Vladimir Putin said in September that if Western countries allow Ukraine to strike deep inside Russia with their longer-rage weapons, "it will mean that NATO countries, the US, European countries are at war with Russia."
“And if it is the case, then, bearing in mind the change of the very essence of the conflict, we will be making appropriate decisions based on threats that will be posed to us,” Putin said.
Russia has recently escalated air attacks, launching complex combined drone and missile barrages to target energy infrastructure this week as temperatures begin to drop.