A Chinese vessel pointed a type of radar normally used to help guide missiles at a Japanese navy ship near disputed East China Sea islets, prompting the Japanese government to lodge a protest with China, Japanese public broadcaster NHK said on Tuesday.
The incident happened on Jan. 30, the defence minister later said.
A long-simmering row over the islands, known as the Senkaku in Japan and the Diaoyu in China, has in recent months escalated to the point where both have scrambled fighter jets while patrol ships shadow each other in nearby seas.
Concern that the increasing cat-and-mouse encounters between aircraft or ships will cause an accidental clash are giving impetus to efforts to dial down tension, including a possible leaders' summit.
But while hopes have emerged of a thaw in the chill that began when Japan bought the islands from a private citizen last September, deep mistrust and pumped up nationalism complicated by bitter Chinese memories of Japan's wartime aggression mean any rapprochement would likely be fragile.