The Japanese government said Friday it has confirmed that a submarine spotted near Japanese territorial waters around the Senkaku Islands in the East China Sea the day before was a Chinese naval vessel.

Defense Minister Itsunori Onodera said the submarine surfaced Thursday flying a Chinese flag on its mast. "This unilaterally heightens tensions," Onodera told reporters.

(Photo courtesy of Defense Ministry)

Japanese Vice Foreign Minister Shinsuke Sugiyama lodged a protest with Ambassador to Japan Cheng Yonghua over the submarine's entry into the contiguous zone, telling Cheng the action "is a serious escalation of the situation," according to the Foreign Ministry.

Thursday marked the first known time that a submerged foreign submarine entered the contiguous zone, an area just outside Japanese waters around the Japan-controlled, China-claimed islands.

A Chinese frigate was also observed Thursday entering the zone, prompting Japan to protest that action.

China claimed the approach was justified by its "full historical and legal basis for its sovereignty" over the islands, which it calls Diaoyu.