Three Chinese patrol vessels entered what Japan says are its territorial waters near islands at the centre of a rift that prompted China to put off an event marking bilateral ties.
The three boats were spotted today close to the islands, known as Senkaku in Japanese and Diaoyu in Chinese, Chief Cabinet Secretary Osamu Fujimura told reporters in Tokyo. The ships later left the area, Japan's Coast Guard said.
"We have protested this incursion at a high diplomatic level," Mr Fujimura told reporters in Tokyo.
At least six Chinese patrol vessels in the area have entered the waters this month. Japan's purchase of the islands this month set off protests and attacks on Japanese businesses in China, harming a $340 billion (€263 billion) trade relationship between Asia's two biggest economies.
Chinese defense minister Liang Guanglie said last week that his government reserves the right to take action on the East China Sea chain of islets, calling them "inherent Chinese territory."
China yesterday postponed a reception for the 40th anniversary of the establishment of diplomatic ties with Japan, the state-run Xinhua News Agency reported
The postponement "is extremely regrettable," Mr Fujimura said. "We should hold talks from a broader perspective and move toward a resolution."
Bloomberg