Three surveillance ships were observed around the Japanese-controlled island chain, marking the 20th time Chinese government vessels entered waters surrounding the islands since September, Kyodo reported.
Tensions between Japan and China over competing claims to the islands have worsened since September when the Japanese government announced the nationalization of the islands, called Senkaku in Japan and Diaoyu in China.
In another development, Japan's Coast Guard detained a Chinese fishing boat, accusing it of unauthorized fishing inside Japan's exclusive economic zone, the Chinese Consulate General in Tokyo said.
The boat was seized Saturday near Japan's Kagoshima prefecture, Xinhua reported Monday.
The Chinese Consulate General said an envoy was sent to Kagoshima to visit the boat's captain and eight crew members, all Chinese nationals, and Japan has been asked to ensure their safety and legal rights.
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