Japan lodged a strong protest with China on Jan. 12 after a foreign submarine detected just outside Japanese territorial waters in the East China Sea was confirmed to be Chinese.

The submarine surfaced in international waters northwest of the disputed Senkaku Islands on the afternoon of Jan. 12 and hoisted the Chinese flag, according to the Defense Ministry.

It was the first confirmation of a Chinese submarine moving in the contiguous zone close to the uninhabited islands, which are claimed by both Japan and China.

Shinsuke Sugiyama, administrative vice foreign minister, lodged a protest by phone with Chinese Ambassador Cheng Yonghua.

Defense Minister Itsunori Onodera expressed serious concern about the incident, calling it an “act that unilaterally raises tensions.”

Japan stepped up patrols around the islands after the sovereignty issue became a major impediment to better bilateral ties. Japan controls the islands, which China calls the Diaoyu Islands.

A Maritime Self-Defense Force escort vessel had been tracking the submarine since it was first detected Jan. 10 and subsequently confirmed its nationality, according to the Defense Ministry.

The submarine did not respond to inquiries from the Japanese escort vessel while it was submerged and after it surfaced. It then entered the open sea, apparently headed for China.

A senior Defense Ministry official said the submarine surfaced because the East China Sea is shallow and the craft risked hitting rocks.

Foreign Minister Taro Kono is expected to urge Beijing to refrain from actions that undermine efforts to mend bilateral relations when he meets his Chinese counterpart, Wang Yi, in China during a scheduled visit.

Japan is taking a resolute stand on issues affecting national security while seeking to repair its relations with China, government officials said.