A Chinese naval ship sailed into waters surrounding disputed East China Sea islands for the first time early Thursday, prompting Tokyo to summon the Chinese ambassador to protest, the Japanese government said.
Russian naval ships were also seen in the area around the same time.
A Chinese naval vessel entered waters surrounding the Tokyo-administered isles, called Senkaku in Japan and also claimed as the Diaoyu islands by China, around 00:50 am (1550 GMT Wednesday), according to the Japanese foreign ministry.
It was a 3,963-ton Jiangkai class frigate, spotted by Japan's guided-missile destroyer Setogiri, the Japanese defence ministry said.
Contiguous waters are a 12-nautical-mile band that extends beyond territorial waters. Under international rules, they are not the preserve of any single country, although the resident power has certain limited rights.
"The fact that (China) sent a naval ship to the contiguous waters of our Senkaku Islands for the first time is an act that unilaterally increases tension and our nation is gravely concerned," Chief Cabinet Secretary Yoshihide Suga told a regular briefing.
Relations between Japan and China deteriorated in 2012 when Tokyo "nationalised" some of the islets.
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