The ongoing territorial dispute over a group of uninhabited islands in the East China Sea is overshadowing talks between the region’s main powers at the Asean forum in Cambodia.
Japan has protested to China after three Chinese fishery patrol ships entered waters near the islands.
They are said to have since left the area.
The row is not on a par with a stand-off between the two countries two years ago, but tension has again been rising between Asia’s two largest economies.
Known as Senkaku in Japan and Diaoyu in China, the islands are located near rich fishing grounds and potentially huge oil and gas reserves.
In Tokyo, the Chinese ambassador was summoned over the ships’ appearance. Japan also lodged a complaint in Beijing, where it was abruptly brushed aside.
“The Diaoyu islands and surrounding waters have been Chinese territory since ancient times,” said Chinese foreign ministry spokesman Liu Weimin.
“Chinese fishery patrol boats went into Chinese-administered waters in a routine manner during the fishing off season in accordance with Chinese law.”
Last weekend, Japan’s prime minister Yoshihiko Noda backed an idea from Tokyo’s governor whereby Japan would buy the islands, now privately-owned by Japanese citizens and leased to the central government.
“The Senkaku islands are without a doubt, in terms of history and international law, our territory,” he said. “We are in control of them.”
Senkaku in Japanese, Diaoyu in Chinese… and Tiaoyutai in Taiwanese. Taipei also claims the islands, and a week ago flexed its muscles over the issue.
Activists and fishermen confronted the Japanese coastguard in the area, a historical fishing area also for Taiwan.
There was a small collision but no serious conflict.