Japan said on Friday it had protested to Beijing about what it called China's unilateral development of resources in the East China Sea, saying the activity was continuing despite a 2008 agreement for bilateral cooperation in the area's waters.
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 Self-Defense Forces 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.
Ties between China and Japan, the world's second- and third-largest economies, have long been plagued with a territorial dispute over the Senkaku Islands, a group of uninhabited islets in the East China Sea, as well as the legacy of Japan's wartime aggression.