Japan was forced to scramble fighter jets, after a Chinese military aircraft violated the country's airspace.
Beijing's Y-9 surveillance aircraft hovered over the Danjo islands in Nagasaki prefecture for about two minutes on August 26.
The Japanese Defence Ministry said the Chinese plane was warned, but no weapons were deployed.
Tokyo summoned Beijing's acting ambassador to lodge a “firm protest.” The government spokesperson called it a totally unacceptable violation of Japan's sovereignty, and a threat to its security. Beijing has not issued an official statement in the matter yet.
The incident marked the first incursion by China into Japan's airspace. But the two nations have been involved in a long-standing dispute over islands in the East China Sea, causing confrontations between the Japanese coastguard and Chinese fishing boats.