At a Friday meeting of the House of Representatives' special committee on security legislation, Japanese 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 militarised 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 (SDF) 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.
Japanese Prime Minister Shinzo Abe said in Friday's lower house special committee meeting: "I strongly object to (China) repeatedly going ahead with unilateral development."
Read more at Sydney Morning Herald
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