In the East China Sea, where no official border between the two countries have been drawn, China has constructed more than a dozen gas exploration platforms on the west of the equidistance line between them.

The Japanese foreign ministry said it filed a protest with a senior official at the Chinese embassy in Tokyo after its navy had found a base structure of a new platform being transported there.

“It is extremely regrettable that China is proceeding with its unilateral development activity, even though it is in the west side of the equidistance line between Japan and China,” the ministry said in a statement.

China is one of Japan’s western neighbours.

Ties between China and Japan, the world’s second and third-largest economies, have long been plagued with a territorial dispute over a group of uninhabited islets in the East China Sea as well as the legacy of Japan’s wartime aggression.