Tokyo lodged a protest with Beijing Tuesday as a Chinese warship directed radar against a Japanese military vessel near a group of disputed islets in the East China Sea, reports said.
Defence Minister Itsunori Onodera said the Chinese ship’s fire-control radar was directed at the Japanese ship on January 31.
He said the government also suspected that a Chinese ship had directed radar a Japanese military helicopter on January 19.
He called the incidents "extremely unusual," and said "they could have led to a dangerous situation."
Japan also issued a protest earlier in the day as two Chinese surveillance vessels entered what Tokyo calls its territorial waters around the disputed islets for more than 14 hours on Monday.
"It is extremely regrettable and totally unacceptable that the ships remained in Japanese waters for a long period of time," Chief Cabinet Secretary Yoshihide Suga told a news conference.
The government's purchase of three of the islets in mid-September sparked anti-Japan protests in dozens of Chinese cities and a boycott of Japanese products.
The Senkaku islands are also claimed by China and Taiwan, where they are called the Diaoyu and the Tiaoyutai, respectively.