The U.S. 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 recommended that U.S. citizens in Ukraine's capital be prepared to immediately shelter in the event of an air alert.

The embassy said its closure and attack warning were issued in the context of ongoing Russian missile and drone attacks on Kyiv and anticipated a quick return to regular operations.

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 U.S.-made longer range missiles had been used in a Ukrainian attack that struck a weapons warehouse in the Bryansk region after U.S. President Joe Biden authorized their use.

Russian President Vladimir lowered the threshold for using his nuclear arsenal, with a revised doctrine announced Tuesday permitting a potential nuclear response by Moscow even to a conventional attack on Russia by any nation that is supported by a nuclear power. That could potentially include Ukrainian attacks backed by the U.S.

Western leaders dismissed the Russian move as an attempt to deter Ukraine's allies from providing further support to Kyiv, but the escalating tension weighed on stock markets after Ukraine used American-made ATACMS longer-range missiles for the first time to strike a target inside Russia.

Putin said in September that if Western countries allow Ukraine to strike deep inside Russia with their longer-range weapons, "it will mean that NATO countries, the U.S., 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.

WATCH l Ukrainians cautiously optimistic, but unclear how many ATACMS will be received: 

Ukrainians welcome chance to fire deeper into Russia

After nearly three years of bombardments, many Ukrainians welcome having the ability to strike back deeper into Russian territory as the U.S. is warned not to further escalate the war.

Military analysts say the U.S. decision on the range over which American-made missiles can be used isn't expected to be a game-changer in the war, but it could help weaken the Russian war effort, according to the Institute for the Study of War, a Washington think-tank.

"Ukrainian long-range strikes against military objects within Russia's rear are crucial for degrading Russian military capabilities throughout the theatre," it said.

Russia lowers nuclear threshold as Ukraine launches long-range missiles

After confirming Ukraine struck Russian territory using long-range missiles provided by the U.S., the Kremlin announced it was lowering its threshold for using nuclear weapons.