Despite a recent easing in Sino-Japanese relations, China continues exploration and production of oil and gas resources in a disputed area in the East China Sea according to an August 23 report by the Asia Maritime Transparency Initiative, a project under the US-based Center for Strategic and International Studies (CSIS).

China’s most recent move is to deploy a mobile offshore drilling unit at the northern end of its chain of oil and gas platforms in the maritime area. According to CSIS, those platforms straddle the median line between China’s and Japan’s respective claims.

The observation is based in recent satellite imagery which shows a newly installed jack-up rig, a common type of offshore drilling unit that has been engaged in exploratory drilling at least since June 25. It did not show up in earlier imagery from April 15.

China’s and Japan’s Economic Exclusive Zones (EEZ) in the area overlap and about 40,000 square kilometers of ocean territory is in dispute.

In 1995, China discovered an underwater natural gas field within its EEZ and began extracting it. Japan argued that although the first Chinese rigs were within its territorial waters, the gas field stretches into the disputed area and, therefore, it wanted a share of the resources.

In June 2008, China and Japan agreed to joint explorations in the Chunxiao field, the first in the area to be developed by China. But even so, China and Japan have been at odds over China’s search for oil and gas in an area where sovereignty is contested.

In March 2011, for instance, Japan expressed regret after China’s National Offshore Oil Corporation was producing oil from Chunxiao, known as Shirakaba in Japan. The recent discovery of a new jack-up rig in the area may lead to further disputes.