Japan on Saturday lodged a protest with China after it spotted six Chinese coastguard ships and about 230 fishing vessels sailing near the disputed Senkaku Islands in the East China Sea.
"This is a unilateral act that raises tensions ... and it is unacceptable to us," a Japanese Foreign Ministry official told the Chinese envoy.
The sighting of the Chinese vessels comes less than a month after an international arbitration court rejected China's claims over almost the entire South China Sea, in a case brought by the Philippines. Beijing has refused to recognize the ruling.
The Japan Coast Guard said some of the vessels were equipped with guns, according to The Japan Times.
The development followed a similar incident on Friday when two Chinese Coast Guard vessels and six fishing ships briefly entered Japanese territorial waters near the Senkakus, prompting the ministry to summon the Chinese ambassador.
Vice Foreign Minister Shinsuke Sugiyama on Friday told Chinese Ambassador Cheng Yonghua that the vessels' entry into its waters and their activities were a violation of Japan's sovereignty and was totally unacceptable.
The coast guard vessels entered Japanese waters soon after the Chinese fishing ships did so, the ministry said.
"Japan cannot accept coast guard ships' actions that seemed to have been accompanying Chinese fishing vessels," a Foreign Ministry source explained.
It also said this was the first time both types of vessels had entered the waters at the same time.
The islets are administered by Japan but claimed by both China and Taiwan, which call them Diaoyu and Tiaoyutai, respectively.
The incident came as tensions between the two countries has escalated in the East China Sea.
In June, Chinese warships were seen traversing areas in and near Japanese waters in the East China Sea and the western Pacific, including near the Senkakus.
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