NATO considers close cooperation between Russia and China to be a "serious challenge"

“We see Russia and China working more and more together, both politically and militarily,” he said.

According to NATO's observations, both countries "are increasingly coordinating their decisions in multilateral organizations," such as the UN.

“In addition, the two countries conduct joint maneuvers, long-range combat aircraft and maritime operations, and actively exchange experience in the use of combat systems and control over the Internet,” Stoltenberg said.

NATO is also concerned about close cooperation between Moscow and Minsk in recent months, the head of the alliance said.