The Japanese government has summoned China's ambassador to Tokyo to protest against four Chinese maritime surveillance ships entering the disputed waters near a group of islands that both countries claim.
The uninhabited outcroppings in the East China Sea, called Senkaku in Japanese and Diaoyu in Chinese, are controlled by Japan but also claimed by China and Taiwan.
Akitaka Saiki, Japan's deputy foreign minister, summoned Cheng Yonghua, the Chinese diplomat to the foreign ministry after the four Chinese government ships remained in the waters off the islands for about 13 hours on Monday night and Tuesday morning, the Japanese government said.
Tensions over the tiny islands intensified after Japan 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 December 13, a Chinese government aeroplane flew into airspace above the islands, prompting the Japanese military to scramble fighter jets and the government to lodge a formal protest.
China said the aircraft was conducting a normal operation.