TOKYO: The Japanese government summoned the Chinese ambassador Tuesday to protest four Chinese maritime surveillance ships that steamed around in waters for half a day near disputed islands claimed by both countries.

The uninhabited outcroppings in the East China Sea, called Senkakus in Japanese and Diaoyu in Chinese, are controlled by Japan but also claimed by Beijing and Taiwan.

Deputy Foreign Minister Akitaka Saiki called Ambassador Cheng Yonghua to the Ministry of Foreign Affairs after the four Chinese government ships remained in waters off the islands for about 13 hours Monday until just after midnight early Tuesday, said Masaru Sato, a ministry official.

Tensions over the tiny islands intensified after Tokyo bought the islands from their Japanese private owners in September, prompting Chinese to hold demonstrations and boycott Japanese products.

The latest incident marked the 21st and longest intrusion of Chinese vessels into what Japan considers its territorial waters around the islands since that purchase, the coast guard said.

On Dec. 13, a Chinese government airplane flew into airspace above the islands, prompting Tokyo to launch a formal protest. Beijing said the plane was conducting a normal operation.