According to the Kyodo news agency, this is the 37th case of Chinese patrol ships entering into the territorial waters of Japan this year.
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 Japanese maps from 1783 and 1785 designating the islands as Chinese territory.
After World War II, the Senkaku Islands were placed under the control of the United States and were later passed to Japan in 1972. Japan believes that China's sovereignty claims over the islands follow the discovery of valuable minerals in their shelf water in the 1970s.
The territorial dispute escalated in 2012, when the Japanese government purchased three of the five islands from a private owner, thus establishing Tokyo's sovereign ownership.