Since the beginning of the war in Ukraine, groups linked to the Chinese government have repeatedly hacked Russian companies and government agencies in an apparent search for military secrets, according to cyberanalysts.

The intrusions started accelerating in May 2022, just months after Moscow’s full-scale invasion. And they have continued steadily, with Chinese groups worming into Russian systems even as President Vladimir V. Putin of Russia and President Xi Jinping of China publicly professed a momentous era of collaboration and friendship.

The hacking campaign shows that, despite this partnership and years of promises not to hack each other, China sees Russia as a vulnerable target. In 2023, one group, known as Sanyo, impersonated the email addresses of a major Russian engineering firm in the hunt for information on nuclear submarines, according to TeamT5, a Taiwan-based cybersecurity research firm that discovered the attack last year and linked it to the Chinese government.

China is far wealthier than Russia and has plenty of homegrown scientific and military expertise, but Chinese military experts often lament that Chinese troops lack battlefield experience. Experts say that China sees the war in Ukraine as a chance to collect information about modern warfare tactics, Western weaponry and what works against them.

“China likely seeks to gather intelligence on Russia’s activities, including on its military operation in Ukraine, defense developments and other geopolitical maneuvers,” said Che Chang, a researcher with TeamT5.

It is unclear how successful these attempts have been, partly because Russian officials have never publicly acknowledged these intrusions. But a classified counterintelligence document from Russia’s domestic security agency, known as the F.S.B., makes clear that intelligence officials are concerned. The document, obtained by The New York Times, says that China is seeking Russian defense expertise and technology and is trying to learn from Russia’s military experience in Ukraine. The document refers to China as an “enemy.”