US Embassy in Kyiv warns of 'potential significant' air attack

LONDON -- The U.S. Embassy in Kyiv temporarily closed its doors on Wednesday out of "an abundance of caution," as it warned that a potentially "significant" air strike may be coming.

"The U.S. Embassy recommends U.S. citizens be prepared to immediately shelter in the event an air alert is announced," the embassy said in an update.

The embassy said it had received "specific" information about the potential strike on Nov. 20.

The warning arrived amid fears that the war could escalate, a day after Ukraine's military first launched U.S.-made long-range ATACMS missiles toward targets within Russia. U.S. President Joe Biden had given Kyiv permission for those strikes, U.S. officials told ABC News.

That strike, which Russia said it had defeated, came on the same day Russian President Vladimir Putin signed an updated nuclear doctrine. A Kremlin spokesperson said the updates meant "the use of Western non-nuclear rockets by the Armed Forces of Ukraine against Russia can prompt a nuclear response."

A U.S. Department of Defense spokesperson sought to downplay that update on Tuesday, saying Russia had been signalling that it would update the doctrine for "the last several weeks."

"It's the same irresponsible rhetoric that we've seen before and that we've seen frankly for the past two years," said Sabrina Singh, deputy Pentagon press secretary, according to a transcript. "So, it's something that we're going to continue to monitor, but we don't have any indications that Russia is preparing to use a nuclear weapon within Ukraine."

She added that Russia has undertaken "escalatory action" by bringing North Korean soldiers onto the battlefield within Russia.

ABC News' David Brennan and Patrick Reevell contributed to this report.