Officials in Tokyo say a Chinese spy ship entered Japanese waters early Wednesday.
The intelligence ship was spotted around 3:30am local time Wednesday near Kuchinoerabu island in southern Japan. The ship reportedly left Japanese waters 90 minutes later.
China claims it was acting within its rights according to international law and freedom of navigation.
The area in question divides the Pacific Ocean and the South China Sea, and is not part of the heated territorial dispute over islands in the South China Sea.
Japan has previously warned that any foreign naval vessels entering its claimed waters for any reason other than “innocent passage” would be instructed to depart by Japanese patrols.
Last week Japan expressed a strong vow to protect its territory following the passage of a Chinese naval vessel close to islands claimed by both countries.
Japan's defense ministry says a Chinese ship was spotted entering a protected zone northeast of Kuba island, part of the islands known as Senkaku in Japanese, and Diaoyu in China.
Chief Cabinet Secretary Yoshihide Suga told reporters in Tokyo that Japan will protect the Senkaku islands “by any means," and criticized China for "unilaterally escalating tensions."
But a spokesman for China's defense ministry says it has every right to sail through those waters.
The uninhabited islands are claimed by China and Taiwan, but controlled by Tokyo.
The islands and rocks are in a strategically significant position in shipping lanes, as well as desirable fishing grounds and are near potential oil and gas reserves.