Japan says it scrambled fighter jets on Sunday after eight Chinese military aircraft flew between Japanese islands.
The planes, thought to be bombers, surveillance planes and one fighter jet, flew along the Miyako Straits, between Okinawa and Miyakojima.
China said about 40 of its aircraft had been involved in what it said was a routine drill.
The planes did not cross into Japanese airspace, but the move is being seen as a show of force by China.
It comes one week after Japan said it would take part in joint training exercises with the US navy in the South China Sea.
Japan's top government spokesman said Japan would be watching China's military movements closely.
Tokyo 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", said Yoshihide Suga.
The Miyako Strait is a strategically important 250km (155 miles) wide stretch of water south of Okinawa and close to Taiwan.
It is also close to a group of islands in the East China Sea which are claimed by both Japan and China.
Japan, which controls the islands, calls them Senkaku, while China calls them the Diaoyu Islands.