Japan says a Chinese warship last week locked its pre-firing radar on a Japanese navy boat near a group of disputed islands in the East China Sea.
The Japanese defense ministry has lodged a complaint with China over the January 30 incident, the latest in a series of dangerous escalations in their long-running territorial dispute.
Defense Minister Itsunori Onodera Tuesday called China's move "very abnormal." He said the radar aimed at the Japanese ship is normally used to help guide missiles.
Although there have not been any major clashes, China has repeatedly sent government ships near the Japanese-administered islands. Analysts say the patrols are meant to establish de facto control of the area.