This Chinese navy survey vessel entered Japan's territorial waters southwest of Yakushima, Sunday, Feb. 12, 2023. (Japan Ministry of Defense)

CAMP FOSTER, Okinawa — Tokyo lodged a diplomatic complaint with Beijing on Sunday after a Chinese navy vessel entered Japan’s territorial waters off the southern tip of its main islands, according to Japanese government statements.

A Shupang-class survey ship crossed the 12-mile territorial limit around Yakushima, an island 40 miles south of Kyushu, the southernmost of Japan’s four main islands, at approximately 2:30 a.m. Sunday, Japan’s Defense Ministry said in a statement that day.

The vessel remained for slightly less than two hours before exiting to the southwest of neighboring Kuchinoerabu island, the statement said.

The survey vessel’s intrusion was the first of the year by the Chinese navy, according to the Ministry of Defense website. Shupang-class survey vessels made five intrusions in 2022, according to the ministry.

Japan’s Foreign Affairs Ministry lodged a complaint with the Chinese Embassy in Japan and with the government in Beijing on Sunday, expressing “strong concern” over the incident, a ministry spokesman told Stars and Stripes by phone Monday.

Some government officials in Japan are required to speak to the media only on condition of anonymity.

The vessel’s intent was unclear, but Japanese authorities were concerned that China has stepped up military activities in the region, Japan’s Chief Cabinet Secretary Hirokazu Matsuno said Monday.

“We will keep paying close attention to the sailing of Chinese navy vessels in our waters and take all the possible measures to conduct vigilant surveillance in the surroundings of our country,” Matsuno said at a press conference in Tokyo, according to a video clip uploaded to Prime Minister Fumio Kishida’s website.

Japan’s Maritime Self-Defense Force spotted the survey vessel south of Yakushima in the country’s contiguous waters, a maritime zone that extends 12 nautical miles beyond the territorial limit, around 12:50 a.m. Sunday, according to the ministry statement.

The vessel then entered Japan’s territorial waters southwest of Yakushima, the ministry said. A P-1 maritime patrol aircraft from Fleet Air Wing 1 at Kanoya Air Base, a P-3C surveillance plane from Fleet Air Wing 5 at Naha and the JS Shirataka, a guided-missile destroyer from Sasebo Naval Base, tracked the Chinese vessel.

Even warships of one country are permitted “innocent passage” through another country’s territorial waters under the U.N. Convention on the Law of the Sea. The convention spells out exceptions to the rule.

The Chinese vessel exited Japan’s territorial waters southwest of neighboring Kuchinoerabu at 4:10 a.m., according to the ministry. Kuchinoerabu is less than 10 miles northwest of Yakushima.

The incident comes amid the fallout from the U.S. shooting down a suspected Chinese spy balloon over the Atlantic on Feb. 4, leading to a series of diplomatic snubs and charged rhetoric by both countries.

The USS Nimitz Carrier Strike Group and the 13th Marine Expeditionary Unit took part in “integrated expeditionary strike force operations” over the weekend in the South China Sea, a 7th Fleet statement said Sunday.