Four Chinese patrol boats, one of them equipped with an automatic cannon, reportedly entered the territorial waters off Uotsuri Island and Minamikojima Island on 30 August. Three more vessels are said to be in the contiguous zone.
Five Japanese fishing boats are currently operating in the area. The Chinese vessels tried to approach them, NHK said.
Prior to that, two Chinese patrol ships entered Japan's territorial waters near the Senkaku Islands on 28 August.
In recent years, Tokyo has repeatedly voiced concerns over Beijing's activities in the region, especially with regard to the disputed Senkaku Islands, known in China as the Diaoyudao Islands. Tokyo claims to have had sovereignty over the islands since 1895, while Beijing argues that they were marked as Chinese territory on Japanese maps dated 1783 and 1785.
After World War II, the United States controlled the islands until handing them over to Japan in 1972. China claims that the islands were illegally seized by Japan. Tokyo, for its part, suggests that China only became interested in the islands in the 1970s because the nearby territory was found to potentially hold oil reserves.