Japan has warned China against expanding its military activity to the skies over disputed East China Sea islands after Chinese warplanes flew near the area over the weekend.
China's Defense Ministry said Monday it had flown more than 40 bombers and other fighter planes through a strait between Japanese islands on their way to drills in the Western Pacific.
The overflights came after Tokyo announced it may patrol alongside the US in the disputed South China Sea.
Chief Cabinet Secretary Yoshihide Suga said Japan scrambled at least one fighter jet after the Chinese planes were spotted passing over Miyako Strait.
The strait lies east of the disputed Japanese-claimed Senkaku islands which China also claims and calls them the Diaoyu.
Suga said the Chinese aircraft did not violate Japanese airspace during the long-distance roundtrip flight, but it was the first time Chinese warplanes had flown that route.
He said Japan would respond firmly to any violation of Japanese air space in the future.
The move came after Japan said earlier this month that Tokyo would increase its engagement in the South China Sea through joint training cruises with the US Navy.
Japan and China have been in a long-time dispute over some uninhabited islets in the East China Sea.
Similar disputes exist between Beijing and other regional countries such as Taiwan, Brunei, Vietnam, Malaysia and the Philippines over territories the South China Sea.
In these disputes, which have become a source of tensions between China, regional countries and the US, Washington has shown support for Beijing's rivals.
The seas are important global trade routes and hold huge mineral deposits.
Beijing asserts sovereignty over almost all of the South China Sea, dismissing rival partial claims from its Southeast Asian neighbors and ejecting any intervention by Japan in the waterway.