Japan called on China on Tuesday to withdraw its coast guard ships from waters near the disputed Senkaku islets in the East China Sea, Tokyo said.
Foreign Minister Fumio Kishida summoned the Chinese ambassador to Japan, Cheng Yonghua, to issue a strong protest as more than a dozen Chinese coast guard ships were seen near the uninhabited islets, according to the Japanese Foreign Ministry.
"The Chinese side is attempting to change the status quo in a one-sided manner. I cannot help saying that because of this, the situation surrounding Japanese-Chinese relations has markedly deteriorated," Kishida told the Chinese ambassador, the ministry reported.
Fifteen Chinese ships were spotted in the area on Monday and three of the boats entered what Tokyo considers its territorial waters, Japanese authorities said.
The Japanese-administered Senkaku Islands are also claimed by China and Taiwan, where they are known as Diaoyu and Tiaoyutai, respectively.
Cheng reiterated the islets are part of China's inherent territory, so Chinese vessels have a right to navigate in the waters, he said, according to local broadcaster NHK.
The ambassador also told reporters after the meeting that both sides should work calmly to prevent further complicating the situation, NHK said.
Tuesday’s complaint against Beijing followed another protest on Sunday after 13 Chinese coast guard ships were seen near the islets.
On Saturday, about 230 Chinese fishing boats and six coast guard vessels were sailing in a contiguous zone near the islets, which prompted Tokyo to lodge a protest.
China's recent moves come after Prime Minister Shinzo Abe last Wednesday picked Tomomi Inada as new defence minister. Inada has controversially denied Japan's Nanjing Massacre of 1937.
In September 2012, Tokyo purchased three of the islets from a private owner, which set off protests in dozens of Chinese cities and a boycott of Japanese products.