Japan scrambled fighter jets near Okinawa after a Chinese military aircraft flew through international airspace near Japan's southern islands.
"I believe this indicates China's move toward further maritime expansion," said Japanese Defense Minister Itsunori Onodera.
Japan's Defense Ministry said a Chinese Y-8 airborne early warning plane flew out over the Pacific, through airspace between Okinawa prefecture's main island and the smaller Miyako island in southern Japan.
China and Japan have been in a territorial dispute over uninhabited East China Sea islets. Last year, tensions rose when Japan bought three of the disputed islands from a private Japanese owner.
Aircraft and ships from both countries have been occupying space around the disputed territories.
Japanese Prime Minister Shinzo Abe, after his Liberal Democratic Party's victory in last weekend's elections, said that Japan would be open to dialogue with China, though he previously vowed to take a firm stance on the territorial dispute.