For example, in 2016, for the first time in years, a PLAN frigate entered the contiguous zone of the Senkaku Islands.
China claimed the approach was justified by its "full historical and legal basis for its sovereignty" over the islands, which it calls Diaoyu.
Japan said the large warship entered contiguous waters around the islands at 11:00 local time (02:00 GMT), and a submarine was also detected there, although neither entered Japanese territory.
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Japan said it had summoned China's ambassador and "issued a protest by expressing serious concerns".
Tokyo purchased three of the islets from a private owner in September 2011, sparking anti-Japan protests in dozens of Chinese cities and a boycott of Japanese products.
"Senkaku is Japan's territory and territorial waters internationally and historically and Japan's claim about the contiguous zone is absolutely correct", Defense Minister Itsunori Onodera told reporters Friday morning. They say both left the contiguous zone on Thursday afternoon. The incident was the first to involve a submarine in the contiguous zone around the Senkakus.
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The Japan Self-Defence Forces - the country's military - spotted the Jiangkai II-class frigate at around 11 a.m., close to the Senkaku islands, a group of uninhabited islands controlled by Japan, although it did not enter Japanese territorial waters, Efe news agency reported.
Onodera made the statement during a visit to Hawaii Thursday. Japan's Chief Cabinet Secretary Yoshihide Suga warned China "not to undermine the trend of improvement in Japan-China relations". A defense ministry spokesman declined to discuss the measures being taken.
China Coast Guard vessels have frequently entered territorial waters around the Senkakus as part of China's claim to the islets.
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Meanwhile, Beijing's foreign ministry spokesman Lu Kang said China had been conducting "surveillance over the activities of the Japanese side", and said that the islands belonged to China.
Maritime confrontations and jet interceptions between China and Japan have become commonplace in recent years, especially around the Senkakus.
Though the Japanese Defense Ministry has not said the submarine was Chinese, the circumstances suggest that it was likely a PLAN vessel.
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The presence of a submarine during the latest incident adds a new dimension to the fairly regular Chinese incursions into these waters.