The U.S. Embassy in Kyiv issued a warning Wednesday about a “potential significant air attack” and announced its closure, following Moscow’s pledge to retaliate after Ukraine launched U.S.-supplied long-range missiles at targets inside Russia for the first time.

“The U.S. Embassy in Kyiv has received specific information of a potential significant air attack on November 20,” the embassy stated on its website.

“Out of an abundance of caution, the Embassy will be closed, and Embassy employees are being instructed to shelter in place,” the message continued, urging U.S. citizens in Ukraine to “be prepared to immediately shelter in the event an air alert is announced.”

The warning comes a day after a senior Ukrainian official confirmed to AFP that U.S.-supplied ATACMS missiles were used in a strike on Russia’s Bryansk region.

Russian Foreign Minister Sergei Lavrov said the attack showed Western countries wanted to “escalate” the conflict.

“This is a direct escalation of the Western war against Russia,” Lavrov said during a press conference at the G20 summit in Brazil. “We will react accordingly.”

U.S. officials confirmed this week that the White House authorized Ukraine to use ATACMS against military targets inside Russia, fulfilling a long-standing request from Kyiv.