The U.S. has warned European allies that Russia asked China for armed drones in late February as it was beginning its invasion of Ukraine, according to people familiar with the matter.

The request has alarmed Biden administration officials who are seeking to prevent China — Russia’s most powerful diplomatic partner — from coming to Vladimir Putin’s aid in the war, according to the people, who described the matter on condition of anonymity.

The Financial Times, meanwhile, reported Monday that the U.S. has told allies that China signaled its willingness to provide military assistance after Russia requested equipment, including surface-to-air missiles, to support its invasion of Ukraine, according to officials familiar with American diplomatic cables on the exchange.