Japan said on Monday it would respond firmly after Chinese government vessels had intruded into what Tokyo considers its territorial waters near disputed islands in the East China Sea 14 times at the weekend, stoking bilateral tensions.
Chief Cabinet Secretary Yoshihide Suga said Tokyo would continue to urge China not to escalate the East China Sea dispute, while also responding firmly and calmly.
Suga told a news conference that a total of 14 Chinese government vessels had entered "contiguous waters", which can be policed for customs and immigration violations, in recent days and intruded into what Japan considers its territorial waters 14 times.
Twelve Chinese vessels remained in the area early on Monday, he said. Agencies including the Coast Guard would act together closely to deal with the situation, Suga said.
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Chinese activity near the islands, known as the Senkaku in Japan and the Diaoyu in China, has heated up since Friday, prompting repeated Japanese protests, including three on Sunday alone.
Some 230 Chinese fishing vessels were also in the area on Saturday, Japan's foreign ministry said.
The incidents come amid heightened tensions after an arbitration court in The Hague invalidated China's sweeping claims in the disputed South China Sea less than a month ago, in a case brought by the Philippines.
China has refused to recognise that ruling. Japan called on China to adhere to the verdict, which it said was binding. This prompted warnings from Beijing to Tokyo not to interfere.
On Saturday, China's foreign ministry spokeswoman Hua Chunying said in a statement on the ministry's website China had indisputable sovereignty over the islands and nearby waters.
China accused Japan's new defence minister, Tomomi Inada, on Friday of recklessly misrepresenting history after she declined to say whether Japanese troops had massacred civilians in China during World War Two.
Inada echoed Suga's comments and said on Monday Japan's military would conduct air patrols to provide information to its Coast Guard.
Ties between China and Japan, the world's second and third-largest economies, have been plagued by the territorial row, the legacy of Japan's wartime occupation of parts of China and regional rivalry.