China and Russia conducted joint air force patrols over the Sea of Japan and the East China Sea on Tuesday, as South Korea said it had deployed fighter jets in response to warplanes near its airspace.
Beijing and Moscow "staged the sixth joint aerial patrol in accordance with an annual military cooperation plan between China and Russia," the Chinese defense ministry said in a statement.
The statement gave no further details of the maneuvers, which took place over waters bordering Japan, the Korean peninsula and Taiwan.
South Korea said four Russian and four Chinese military aircraft had entered its air defense identification zone (ADIZ) around lunchtime on Tuesday, prompting it to scramble fighter planes.
An ADIZ is an area wider than a country's airspace in which it tries to control aircraft for security reasons, but the concept is not defined in any international treaty.
The South Korean military "identified the Chinese and Russian jets before their entry into the air identification zone," Seoul's Joint Chiefs of Staff (JCS) said.
"We deployed air force fighters to conduct tactical steps in preparation in case of an emergency," it added.
The eight foreign jets did not violate Seoul's airspace, the JCS said.
The incident comes after the defense ministers of South Korea, Japan and the United States on Saturday agreed to set up real-time data sharing on North Korean missile launches by the end of the year.
South Korea has supported Western sanctions on Russia over the war in Ukraine and has sent humanitarian aid to Kyiv.
But it is bound by its own regulations prohibiting the supply of lethal aid to countries in conflict.
China says it is a neutral party in the Ukraine war. It has been criticized by Western countries for refusing to condemn Moscow and for its close strategic partnership with Russia.
In February, Beijing released a paper calling for a "political settlement" to the conflict, which Western countries warned could enable Russia to hold much of the territory it has seized in Ukraine.
During a March summit in Moscow, Chinese leader Xi Jinping invited Russian President Vladimir Putin to visit Beijing and the two leaders declared that ties were "entering a new era."
Analysts say China holds the upper hand in the relationship with Russia, and that its sway is growing as Moscow's international isolation deepens.
Beijing and Moscow were Cold War allies with a tempestuous relationship but in recent years have drawn closer, especially in the economic and military realms.
Both countries have touted what they call a "no-limits" partnership and played up what they frame as a close friendship between presidents Xi and Putin.