According to NHK broadcaster citing Japan Coast Guard officials, the Chinese ships entered the waters of Minami Kojima island about 7 a.m. local time [22:00 GMT Sunday], navigating some 13 miles southeast from the island.
The Japanese authorities sent patrol ships to the site to ensure security of a Japanese fishing vessel that was approached by the Chinese ships, the report said.
The incident is the seventh time when China has infringed on Japan's territorial waters since the beginning of the year. The previous case was detected on 12 April.
Both Japan and China have extended territorial claims over the Senkaku Islands. Japan insists on its sovereignty over the islands, effective since 1895, while China points to the 1783 and 1785 Japanese maps designating the islands as Chinese territory. After World War II, the Senkaku Islands went under the control of the United States and were passed on to Japan in 1972.
Japan believes that China's sovereignty claims over the islands follow the discovery of valuable minerals in their shelf waters in the 1970s. The territorial dispute escalated in 2012, when the Japanese government purchased three of the five islands from a private owner, thereby confirming Japan's affiliation with the islands.