Two Chinese government ships on Monday entered waters around Tokyo-controlled islands that Beijing claims as it own, the Japanese coastguard said.
The maritime surveillance boats arrived in the waters around what Japan calls the Senkakus and China calls the Diaoyus shortly before 9:30 am (0130 GMT).
Beijing has repeatedly sent ships to the area since Japan nationalised some islands in the chain in September. The move triggered a diplomatic dispute and huge anti-Japan demonstrations across China.
Beijing has also sent air patrols to the archipelago and recently both Beijing and Tokyo have scrambled fighter jets, though there have been no clashes.
Voices on both sides have tried to soothe ruffled relations, with some senior politicians suggesting the dispute be set aside for "future generations" to resolve.
Japanese Prime Minister Shinzo Abe has stressed that there is no room for Tokyo to compromise over the territorial row, but has expressed his wish to meet with his Chinese counterpart to improve bilateral ties.