China has been constructing a new offshore platform near the median line between its shoreline and that of Japan in the East China Sea, where China is developing gas fields, according to the government.
At a Friday meeting of the House of Representatives' special committee on security legislation, Defence Minister Gen Nakatani said the move could become a security concern.
China "could install a radar system on the platform, or use it as an operating base for helicopters or drones conducting air patrols," he said.
China has been reclaiming rock reefs in the Spratly Islands in the South China Sea to build an airfield and communications facilities, so this latest discovery could indicate progress toward creating a militarized base in the East China Sea.
The Japanese government is regularly monitoring this area of the sea via air patrols by the Self-Defence Forces and Coast Guard, though it did not offer information on how many new platforms are being built, their locations, their scale or other details.
It became evident in June 2013 that China was building an offshore platform in the area.
The Japanese government demanded that construction be stopped, but it appears that more work has been done.
Prime Minister Shinzo Abe said in Friday's lower house special committee meeting, "I strongly object to [China] repeatedly going ahead with unilateral development."
In November 2013, China declared an air defence identification zone (ADIZ) in the East China Sea, though it cannot completely cover this area with radar from the Chinese mainland.
"If radar was installed on the platform, it would supplement the [ADIZ]," Nakatani told the committee. "It would improve China's capacity for surveillance and warning, which could give them a better idea of the SDF's activities."
The boundary between Japan and China's EEZs in the East China Sea is not clearly demarcated.
The area where the platform is being built is on China's side of a line that runs midway between the two nations' shores.
In 2004, it came to light that China was unilaterally developing four gas fields in this area, at Shirakaba (Chinese name: Chunxiao), Kashi (Tianwaitian), Asunaro (Longjing) and Kusunoki (Duanqiao).
The Japanese and Chinese governments in June 2008 agreed to establish a joint development region for sea areas that straddle the midline, and to co-operate on developing the Shirakaba gas field, which sits on the midline.
Talks over how to deal with the other three fields have been ongoing, but little progress has been made.