Japan lodged a protest with China on Sunday after two Chinese coastguard vessels entered what Tokyo considers as its territorial waters near a group of disputed islets in the East China Sea, Japanese authorities said.
Japanese vice foreign minister Shinsuke Sugiyama called the Chinese ambassador to Japan, Cheng Yonghua, telling him the activities were in violation of Japanese sovereignty and unacceptable, according to the Foreign Ministry.
The two ships were among 13 Chinese coastguard ships in a contiguous zone off the Japanese-administered Senkaku Islands, broadcaster NHK reported.
The uninhabited islets are also claimed by China and Taiwan, where they are called Diaoyu and Tiaoyutai, respectively.
Sunday's complaint by Tokyo followed a another protest against Beijing on Saturday after six Chinese coast guard ships and 230 Chinese fishing boats were spotted in waters near the islets.
On Friday, the government summoned Cheng to lodge a protest after eight Chinese fishing boats and coast guard ships approaching the Senkakus.
China's recent moves come after Japanese Prime Minister Shinzo Abe on Wednesday named Tomomi Inada as the country's new defence minister. Inada has controversially denied Japan's Nanjing Massacre in 1937.