Russian President Vladimir Putin and Chinese leader Xi Jinping oversaw an ambitious joint military exercise in China this summer, which along with reported collaborations in aviation, undersea and hypersonic-weapons technology point to a solidifying defense alignment, according to military analysts.

U.S. officials and military specialists say it is difficult to pin down the level of collaboration between two nations that tightly control information, and whose actions are increasingly opaque to outsiders. But Western officials and defense experts are growing more convinced of the closer relationship based on recent economic alliances, military exercises and joint defense development, as well as the few public statements from government leaders.

While U.S. officials have long been skeptical of a unified threat from the two countries, some are now changing their tune. The Office of the Director of National Intelligence reported that Beijing and Moscow are now more aligned than at any point in the past 60 years.

“They are distinct threats. But they are now interrelated because of the collaboration,” said Michael Kofman, a Russian military expert at CNA, a nonprofit research and analysis group in Arlington, Va. Biden administration officials say they are watching closely but caution against reading too much into actions unlikely to flower into a full military alliance.

China and Russia, which share a 2,500-mile border, have competing interests in Central Asia, India and the Arctic that prevent a complete alignment. And unlike countries in the North Atlantic Treaty Organization, defense systems in China and Russia aren’t broadly compatible, limiting the effectiveness of a joint command.

Yet U.S. steps to contain the two countries have driven them into a marriage of convenience, giving the previously contentious rivals an incentive to marshal resources and intelligence against a common adversary, according to analysts and U.S. officials. China and Russia are eager to restrain U.S. influence as well as its military and financial potency, which they believe more likely if they work in tandem, according to analysts.

Chinese Foreign Minister Wang Yi said in June that the relationship between China and Russia was poised to reach a “larger scale, broader field, and deeper level.” Mr. Putin said relations with China were at historic levels.

While Washington grappled with the collapse of Afghanistan, Chinese and Russian troops in August executed military drills in northwestern China. The exercises were the first to use a joint Russian-Chinese command-and-control set up, signaling a growing ability to coordinate in any potential action against the U.S., according to analysts.

Roughly 13,000 troops and hundreds of aircraft, drones, artillery pieces, antiaircraft batteries and armored vehicles gathered in the Ningxia region. China’s minister of national defense, Wei Fenghe said the military exercises showed a “high level of development of inter-army ties.” In October, the two countries launched joint naval drills off Russia’s Far East coast.