Photo/Illutration A Chinese naval vessel enters the contiguous zone southwest of Uotsurishima island in Okinawa Prefecture on July 4. (Provided by the Japanese Defense Ministry)

A Chinese frigate was spotted passing through the contiguous zone around the disputed Senkaku Islands in Okinawa Prefecture, the government announced on July 4.

This was the fourth time since June 2018 that a Chinese naval vessel has sailed through the contiguous zone near the uninhabited islands controlled by Japan but claimed by China. Beijing calls the islets Diaoyu.

The government said that the frigate didn’t enter into Japan’s territorial waters.

Deputy Chief Cabinet Secretary Seiji Kihara told a news conference on July 4 that a Jiangwei-class frigate entered the contiguous zone at 7:44 a.m. that day and left at 7:50 a.m.

“The Senkaku Islands are an inherent part of our territory historically and under international law,” Kihara said. “We will respond firmly and calmly to resolutely defend our territory, territorial waters, and airspace.”

The Defense Ministry said a Maritime Self-Defense Force escort ship monitored the Chinese frigate. The ministry said the frigate sailed on a west-southwest course after leaving the contiguous zone.

Tokyo said the intrusion “is a grave concern” and lodged a protest with Beijing through diplomatic channels and called for prevention of a recurrence.

The contiguous zone is waters 12 nautical miles (22 km) outside of territorial waters. While coastal nations are allowed a certain degree of control over the waters, they do not have sovereignty there.

Japan has over the years lodged protests to China over repeated intrusions by Chinese Coast Guard ships into Japanese waters around the Senkaku Islands.