The Japanese coast guard has alleged that Chinese vessels, for the second time in a week, sailed into its territorial waters in the disputed East China Sea on Saturday, 12 November. The first such incident took place on 6 November.
The Japanese coast guard said four Chinese coast guard vessels entered the waters around a group of islands, which Japan refers to as Senkaku, while it is known as Diaoyu in China.
The vessels entered at around 10.30am local time (BST) and left within two hours, the coast guard was cited as saying by AFP.
Following the previous incident, Tokyo took the matter up with the Chinese foreign ministry through its embassy in Beijing. Japan conveyed to China that the uninhabited islets are "an inherent territory of Japan".
Both China and Japan claim rights over the waters and islands in the disputed East China Sea. The row has soured diplomatic relations between the two nations, which keep accusing each other of illegally entering the disputed waters.
Japan has also accused China of illegal construction in the disputed islands, backing its claims with satellite images.
Japan has been trying to gather support from its Asian neighbours who too are against China's claims over the South China Sea and the East China Sea. The Philippines, Indonesia and Vietnam have all disputed China's claims over the sea waters, but Filipino President Rodrigo Duterte was recently seen moving closer to its former rival China, leaving the Japanese administration concerned.