Photo/Illutration The Kagoshima Prefecture islands of Yakushima, in the background, and Kuchinoerabujima (Asahi Shimbun file photo)

A Chinese naval vessel intruded into Japanese territorial waters on Nov. 17, the first such incident in about four years, prompting an official complaint to Beijing passed on through diplomatic channels, the Defense Ministry announced.

Ministry officials said the survey ship must have “intentionally intruded” into sovereign waters on the evening of Nov. 17 off the coast of Yakushima island of Kagoshima Prefecture, south of the main southern island of Kyushu.

Chinese Coast Guard ships have repeatedly entered territorial waters in the southwestern portion of the Japanese archipelago, particularly near the disputed Senkaku Islands of Okinawa Prefecture. However, it is rare for the Defense Ministry to formally announce that a Chinese naval ship had entered territorial waters.

The last such incident occurred in July 2017.

Areas within about 22 kilometers of the coastline are considered territorial waters, while those stretching roughly 22 km further afield are known as the contiguous zone.

According to Defense Ministry officials, a Maritime Self-Defense Force P-1 patrol aircraft spotted the Chinese naval survey ship moving south of Yakushima in a westward direction in the contiguous zone and heading for territorial waters around 8:40 p.m. on Nov. 17. 

Around 1:20 a.m. on Nov. 18, the vessel was moving westward in the contiguous zone off Kuchinoerabujima island, which is also a part of Kagoshima Prefecture.

That led Defense Ministry officials to conclude the survey ship must have intruded into Japan’s territorial waters at some point.

The only other times the Defense Ministry has announced intrusions into territorial waters by Chinese naval ships were in November 2004 and June 2016.

“This latest incident was highly unusual because we concluded that the ship intentionally intruded into our territorial waters,” a Defense Ministry official said.

Under the U.N. Convention on the Law of the Sea, innocent passage through territorial waters is permitted as long as the foreign ships do not engage in military or economic activities. Defense Ministry officials said they were still determining if that applies to the latest incident.