According to the broadcaster NHK, citing Japanese Coast Guard officials, Chinese vessels entered into Japan's territorial waters at 9:45 a.m. local time (00:45 GMT) and remained there for two hours. Then, the Chinese vessels were located in a contiguous zone, approximately 24-31 kilometres (38-49 miles) north of Senkaku's Kubashima Island.
Today's incident is the sixth time China has entered the disputed area since the beginning of the year. The last incident was detected on 16 March.
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 came 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 water 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.