TOKYO -- A previously unidentified submarine observed Thursday near Japanese territorial waters off the Senkaku Islands belonged to the Chinese navy, Japan confirmed Friday.
The group of uninhabited islets in the East China Sea has been a source of friction between Tokyo and Beijing, which claims the islands and calls them the Diaoyu.
The day after the submerged vessel was detected, two Japanese Maritime Self-Defense Force destroyers confirmed the submarine had surfaced in the East China Sea northwest of the islets and sailed under a Chinese flag. The destroyers continued to tail the submarine after the confirmation.
It was the first confirmed identification of a Chinese submarine in the Senkakus' contiguous zone, Japan's Ministry of Defense said.
Shinsuke Sugiyama, Japan's vice minister for foreign affairs, complained by phone Friday to the Chinese ambassador to Japan, Cheng Yonghua. Sugiyama reportedly described the incident as a one-sided attempt to shift the status quo involving the Senkakus, and he demanded that measures be taken to prevent a recurrence.
"We are deeply concerned," Defense Minister Itsunori Onodera told reporters the same day. "We need to exercise absolute vigilance" and observe the situation carefully.
China claimed the sub entered the Senkakus' contiguous zone in response to a previous entrance by an MSDF vessel. A source affiliated with Japan's Foreign Ministry said the MSDF ship had only been keeping watch for submarines, and that China bore responsibility for creating the issue.
A Foreign Ministry official said that the government needs to determine whether the Chinese ship received orders from high up or the decision to enter those waters was made on site.