Chinese frigates allegedly aimed weapons-targeting radar at a Japanese destroyer and helicopter in two incidents last month, prompting Tokyo to lodge a protest with Beijing on Tuesday amid tensions over a territorial dispute in the East China Sea.
Analysts described the alleged actions as "provocative" as they could have been a precursor to firing missiles at Japanese forces.
"It is a threatening sign to Tokyo," said Antony Wong Dong, of the Macau-based International Military Association. Lin Chong-pin, Taiwan's former deputy defence minister, said: "It is a ramping up of action by Beijing to show its sovereignty."
Japanese Defence Minister Itsunori Onodera said that "something like fire-control radar was directed at a Japan Maritime Self-Defence Force destroyer in the East China Sea on January 30".
He said similar radar was beamed at a Japanese military helicopter on January 19.
Tokyo released undated pictures of the PLA frigates Linyungang and Wenzhou, which both belong to the East Sea Fleet and were allegedly involved.
Japan's foreign ministry lodged a protest with the Chinese embassy in Japan on Tuesday. The Chinese defence ministry press office was not reachable late yesterday and there was no immediate foreign ministry comment.
Onodera said the delay in announcing the incidents was due to the need to handle the cases carefully, Japanese broadcaster NHK reported, adding that Japanese Prime Minister Shinzo Abe was briefed on Tuesday.
Citing military sources, the Asahi newspaper said the two countries scrambled fighter jets several times north of the Diaoyus on January 19, but no public announcements were made. Hours before the incidents, US secretary of state Hillary Rodham Clinton spoke of opposing "any unilateral actions" seeking to "undermine Japanese administration [of the Senkakus]."
Sino-Japanese ties have deteriorated since September when the Japanese government announced the purchase of three uninhabited islands in the East China Sea - called the Diaoyus in China and Senkakus in Japan. Tensions appeared to have eased in recent weeks after visits to Beijing by a member of Japan's governing coalition and two former Japanese prime ministers.
Ni Lexiong, director of the sea power and defence policy research institute at Shanghai University of Political Science and Law, described the radar incidents as "a warning by the Chinese military to the Japanese vessels". "It is a face-off between the two countries," he said.
Earlier on Tuesday, Tokyo summoned Chinese ambassador Cheng Yonghua after reports that two Chinese surveillance vessels sailed in Japanese waters near the islands on Monday. Cheng rejected the claims.
Additional reporting by Agence France-Presse, Minnie Chan