Chinese government ships returned to waters off disputed Japanese-controlled islands Tuesday, the coastguard said, a week after they last left and days after heated exchanges at the United Nations.
Four maritime surveillance ships entered the waters shortly after 12:30 pm (0330 GMT), where they remained for around six hours before departing.
It was the first time in about a week that Chinese ships had entered the waters, following a lull in a diplomatic row over the sovereignty of the islands known as the Senkakus in Japan and Diaoyus in China.
Foreign Minister Koichiro Gemba said the Japanese government lodged an immediate protest with China over the latest case, telling reporters: "We want the Chinese side to exercise self-restraint."
In Beijing, foreign ministry spokesman Hong Lei said China was "strongly unsatisfied" with moves by "Japanese right-wingers" to enter waters around the islands.
Al Jazeera's Steve Chao reports from Ishigaki in Japan.
