The Diaoyu Islands are a group of uninhabited islands located roughly due east of mainland China, northeast of Taiwan, west of Okinawa Island, and north of the southwestern end of the Ryukyu Islands. They are currently controlled by Japan, which calls them Senkaku Islands. Both China and Taiwan claim sovereignty over the islands.
Abe calls radar-locking incident on high seas 'regrettable and dangerous'
Analysts unsure whether Chinese ships locking radar on Japanese was response or provocation
Teddy Ng and Julian Ryall in Tokyo
Analysts have questioned whether the Chinese frigates that allegedly aimed weapons-targeting radar at a Japanese destroyer and a helicopter last month were being provocative or responding to unannounced threats.
Tokyo said yesterday the incidents were "extremely regrettable and dangerous", but the Defence Ministry in Beijing remained silent on Japan's accusation, first aired on Tuesday.
The latest spat has led to further doubts that the two nations can peacefully resolve their territorial dispute in the East China Sea over the Diaoyu Islands, called the Senkakus in Japan, regardless of Beijing's motives.
Japanese Defence Minister Itsunori Onodera said on Tuesday that a Chinese frigate directed fire-control radar at a Japanese destroyer in the East China Sea on January 30. A similar incident involving a Japanese helicopter occurred on January 19.
Japanese Prime Minister Shinzo Abe said Beijing's move was dangerous and could have led to an unpredictable situation.
Chinese Foreign Ministry spokesman Hua Chunying said yesterday she was "not aware of the specifics" of the incident, and the Defence Ministry did not respond to requests for comment.
Ni Lexiong, of Shanghai University of Political Science and Law, said the Chinese military might have been responding to provocations by Japanese ships.
Tokyo said the Chinese frigate and the Japanese destroyer were three kilometres apart during the January 30 incident. Broadcaster NHK said both incidents occurred on the high seas some 100 kilometres north of the Diaoyus.
Gary Li, an expert on the People's Liberation Army at the London-based private intelligence firm Exclusive Analysis, said that distance between ships was short for large naval vessels, and "sending a warning was a desirable decision by the Chinese captain" if the Japanese ship was too close.
Professor Da Zhigang , from the Heilongjiang Academy of Social Sciences, said it was possible that Beijing was testing how the Japanese navy would handle emergencies.
"The move possibly resulted in a miscalculation of each other's intentions," he said.
The incident has triggered concern in Japan that the Chinese military is running wild and is not eager to resolve the dispute through peaceful dialogue.
"This is an escalation of China's actions and designed to show its coercive power and military capabilities," said Professor Go Ito, an international relations specialist at Meiji University in Tokyo. "And it's not just over the islands; it's to do with overall relations, including the disagreement over the continental shelf, territory within the East China Sea and so on."