Japan was forced to scramble a number of fighter jets to counter eight Chinese military aircraft as they approached Japanese airspace on Sunday, Tokyo has said.
The Chinese planes reportedly flew along the Miyako Straits between Japan’s Okinawa and Miyakojima islands. They were partaking in a military drill including some 40 aircraft nearby.
Although the Chinese planes did not cross into Japanese airspace, they flew over a group of islands in the East China Sea that are disputed by Tokyo and Beijing.
The islands, located between Okinawa and Taiwan, are currently controlled by Japan, which calls them Senkaku. China, which also claims them, calls them the Diaoyu Islands.
However, the exercise took place in what China identifies as its Air Defense Identification Zone, which encompasses the islands. Japan rejects China’s assertion of control of the airspace.
Speaking to reporters in Tokyo on Monday, Japan’s Chief Cabinet Secretary Yoshihide Suga said Japan will "continue to devote every effort to vigilance and surveillance and rigorously enforce steps against intrusions into our airspace based on international law and the self-defence forces law."
China held the exercises a week after Japan and the US agreed to carry out joint military exercises in the South China Sea, where Beijing also contests sovereignty over islands with neighbouring states, including US allies Vietnam, Malaysia and the Philippines.
Last year, Japan announced a plan to deploy about 500 ground troops near the disputed islands after Prime Minister Shinzo Abe's government passed new security laws. The laws were met with harsh criticism from opponents who saw them as a violation of Japan's pacifist constitution.
In March, Japan switched on a radar station on the island of Yonaguni, 150 km south of the Senkaku islands, giving it a permanent intelligence gathering post.
The Japanese cabinet also approved its biggest ever military budget of $41.4 billion for the fiscal year 2016-2017.
Over the next five years, Japan plans to fortify its Self Defence Force in the East China Sea by about a fifth to almost 10,000 personnel, including missile batteries that will help Japan draw a defensive curtain along the island chain.
Chinese ships sailing from their eastern seaboard must pass through this barrier to reach the Western Pacific, access to which Beijing needs both as a supply line to the rest of the world's oceans and for naval power projection.