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."

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.