TOKYO (Kyodo) -- A Chinese frigate and an unidentified foreign submerged submarine were spotted Thursday just outside Japanese territorial waters near the Senkaku Islands in the East China Sea, the Defense Ministry said, prompting Tokyo to protest to Beijing over the sailing.
It is the first known time that a submerged foreign submarine has entered an area just outside Japanese waters, known as a contiguous zone, near the Japan-controlled, China-claimed islands. The ministry believes that the submarine is also from China.
China justified the approach shortly after the protest, claiming it has "full historical and legal basis for its sovereignty" over the islands, which the country calls Diaoyu.
The friction comes despite all the recent positive developments between Asia's two biggest economies, with top political leaders from both sides agreeing to intensify efforts to improve relations.
Chinese Foreign Ministry spokesman Lu Kang, speaking at a press briefing in Beijing, said its navy was in the area as it had to follow and monitor activities of two vessels belonging to Japan's Maritime Self-Defense Force.
According to the Japanese ministry, the submarine in question was first seen Wednesday afternoon in a contiguous zone near Miyako Island elsewhere in Okinawa Prefecture.
It was spotted again in the zone northeast of Taisho Island, part of the Japanese-administrated uninhabited islands, on Thursday morning, as was a Chinese Jiangkai 2-class frigate. Both vessels left the zone in the afternoon, the ministry said.
Japanese Vice Foreign Minister Shinsuke Sugiyama summoned Ambassador to Japan Cheng Yonghua to the Foreign Ministry and protested the frigate's entry into the zone.
Sugiyama expressed "grave concern" and "strongly urged the Chinese side not to undermine the trend of improvement in Japan-China relations," according to the Foreign Ministry.
Although often strained over territorial and historical grievances, bilateral ties have recently shown signs of a thaw, with Japanese Prime Minister Shinzo Abe and Chinese President Xi Jinping agreeing late last year to make a "new start."
The last time a submerged foreign submarine was seen anywhere in Japanese contiguous waters was in February 2016.
The contiguous zone is defined as being within 24 nautical miles of shore, while Japanese territorial waters are within 12 nautical miles from shore. The sailings into the contiguous zone do not present a problem under international law.
China Coast Guard vessels have frequently entered territorial waters around the Senkakus as part of China's claim to the islets.
Kenji Kanasugi, director general of the Japanese Foreign Ministry's Asian and Oceanian Affairs Bureau, also lodged a separate protest with senior officials at the Chinese Embassy in Tokyo, according to a senior Foreign Ministry official.
The Japanese government purchased most of the Senkaku Islands from a private Japanese owner in 2012, putting them under state control. The development prompted sharp criticism from China and triggered frequent intrusions by China Coast Guard vessels into Japanese waters around the islets.
In November 2004, a Chinese nuclear-powered submarine entered Japanese territorial waters around other islands in Okinawa Prefecture.