Chinese military aircraft likely used nearby Japanese destroyers as targets during a missile drill in international waters in the East China Sea in May, Japanese government sources said Sunday.
The Japanese government viewed the drill as "an extremely dangerous military activity" that could have developed into a contingency, the sources said.
But Japan has not lodged a protest with China or disclosed the incident publicly as it does not wish to reveal its intelligence gathering and analysis capabilities, according to the sources.
While relations between Japan and China have been improving recently, the two countries remain at odds over the East China Sea, with Japan repeatedly protesting over unilateral gas field development by China and intrusions in Japanese territorial waters by Chinese vessels.
Japan and China agreed in 2008 on joint gas field development in the East China Sea, but the project has stalled since a collision between a Chinese fishing vessel and a Japanese patrol ship in 2010 off the Japanese-administered Senkaku Islands that are claimed by China, which calls them Diaoyu.
The Chinese aircraft drill indicates that tensions remain in the area, highlighting the necessity to develop a mechanism to avoid accidental clashes, defense experts said.
According to the sources, in late May several JH-7 fighter bombers approached two Maritime Self-Defense Force destroyers that were within striking distance of anti-ship missiles.
The crews of the Japanese vessels were not able to determine the intention of the Chinese pilots, who did not lock on guided missile radar.
However, units of the Japanese Self-Defense Forces intercepted communications between the Chinese aircraft in which the pilots said they would conduct a drill using the MSDF vessels as mock targets, according to the sources.
Based on analysis of the radio communications, the flight paths of the aircraft and other information, the Japanese government came to the conclusion that an anti-ship drill had been conducted.
Some Japanese government officials believe the incident was a provocation, the sources said, while defense experts said the situation needed to be further analyzed.
"Usually, it is not possible for any military organization to use another country's military as a target during a drill in international waters," said Bonji Ohara, a senior fellow at the Sasakawa Peace Foundation, adding it is necessary to figure out whether the move was ordered by Chinse military commanders or just carried out by the pilots.
In 2013, a Chinese warship directed a fire-control radar at an MSDF destroyer in the East China Sea, prompting the Japanese government to lodge a protest with China.