Four Chinese ships briefly entered what Japan considers its territorial waters near disputed islands in the East China Sea on Monday, prompting an official protest from Tokyo and renewed diplomatic efforts to cool tensions between the rivals.

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 defence" 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.

"In recent days, Japan has constantly provoked incidents concerning the Diaoyu islands issue, gravely violating China's territorial sovereignty," Xinhua reported.

The ship patrols were intended to exercise China's "administrative jurisdiction" over the islands, it said.

"Following the relevant laws of the People's Republic of China, (the ships) again carried out a regular rights defence patrol in our territorial waters around the Diaoyu islands."

Sino-Japanese ties have long been plagued by China's memories of Japan's military aggression in the 1930s and 1940s and present rivalry over regional influence and resources.

Japanese Vice Foreign Minister Chikao Kawai will visit China on Monday to discuss Sino-Japanese relations with Chinese Vice Foreign Minister Zhang Zhijun, the Foreign Ministry said.