Ministry spokesman Hong Lei said Wednesday Chinese marine surveillance vessels conduct "regular patrols and law enforcement" in waters surrounding the islands, claimed by both countries, China's state-run news agency Xinhua reported.
A statement issued by China's Oceanic Administration indicated four marine surveillance ships intercepted the Japanese vessels Tuesday while on patrol. Besides taking photographs to document their presence, the fleet radioed the Japanese vessels to emphasize China's sovereignty over the islands and the waters around them.
The number and type of vessels were not reported.
Tensions between Japan and China over competing claims to the islands have worsened since September when the Japanese government announced the nationalization of the islands, called Senkaku in Japan and Diaoyu in China. There have been violent protests in China against Japan's claims, and the Chinese navy has been dispatching surveillance ships to the region, drawing strong Japanese protests.