"The U.S. Embassy in Kyiv has received specific information of a potential significant air attack on November 20," the embassy said in a statement posted on its website.
"Out of an abundance of caution, the Embassy will be closed, and Embassy employees are being instructed to shelter in place," it added.
"The U.S. Embassy recommends U.S. citizens be prepared to immediately shelter in the event an air alert is announced," the statement said.
The unusual warning followed Ukraine's use of US ATACMS missiles to strike Russian territory, marking the first such attack after receiving authorisation from the outgoing administration of US President Joe Biden on the war's 1,000th day, the Reuters news agency reported.
Poland's Foreign Minister Radosław Sikorski said in an X post on Wednesday that Polish diplomats would "remain on duty" despite the heightened risk of Russian attacks on Kyiv.
He described the plan as a sign of solidarity with Ukraine in its fight for freedom.
Russia invaded Ukraine by land, air and sea on February 24, 2022, starting the largest military conflict in Europe since World War II.
Wednesday is day 1,001 of Russia's war on Ukraine.
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Source: IAR, PAP, Reuters, usembassy.gov