In a move that could further complicate the territorial row that is threatening relations between Asia's biggest economies, a group of fishermen from Taiwan -- which also claims the rocky isles -- said as many as 100 boats escorted by 10 Taiwan Coast Guard vessels were headed for the area. 

China's Xinhua news agency said in the morning that two civilian surveillance ships were undertaking a "rights defense" patrol near the islands, citing the State Oceanic Administration, which controls the ships. Two fishery patrol vessels were also detected inside waters claimed by Japan.

  Japan lodged an official protest. 

By afternoon, all four Chinese vessels had moved further away, the Japanese Coast Guard said.

  Sino-Japanese relations deteriorated sharply after Japan bought the islands, called Senkaku in Japan and Diaoyu in China, earlier this month, sparking anti-Japan protests across China.