Russian and Chinese aircraft carried out a joint patrol over far eastern Russia and the Bering Sea near Alaska, in what the Pentagon called the first such joint operation of its kind.
The patrol, which took place on Wednesday and involved nuclear-capable bombers, came shortly after Moscow said the US had sent its strategic aircraft close to Russian airspace.
"[Russian] Tu-95MS strategic missile carriers and the Chinese air force's Xian H-6 strategic bombers carried out an aerial patrol over the Chukchi and Bering Seas and the north Pacific Ocean," Moscow said.
US Defence Secretary Lloyd Austin told journalists on Thursday: "This was not a surprise to us. We closely monitored these aircraft, tracked the aircraft, intercepted the aircraft."
"This is the first time that we've seen these two countries fly together like that," Mr Austin said.
But he said the aircraft did not enter US airspace, with their closest approach being about 320km from the coast.
Norad, the joint US-Canadian North American Aerospace Defence Command, announced on Wednesday that American and Canadian warplanes had intercepted two Russian and two Chinese bombers in international airspace near Alaska.
It said the bombers "remained in international airspace" and were "not seen as a threat".
Moscow said the patrol observed international law and did not breach foreign airspace.
"At certain stages of the route, the aviation group was accompanied by fighter jets of foreign countries," it said.
The patrol was part of "a plan of military co-operation for 2024 and not directed against third countries", Moscow said.
The Chinese Defence Ministry also said that the patrol was "not aimed at a third party" and "has nothing to do with the current international and regional situation".
