Chief Cabinet Secretary Yukio Edano said here that "it is very regrettable" that this kind of report, based on remarks by a named senior official at China National Offshore Oil Corp, has been dispatched and that Japan is asking China to verify whether it is true. Edano said Japan will decide on its response to China after receiving a reply from Beijing.
The gas field in question, called Shirakaba in Japan and Chunxiao in China, is located near what Tokyo claims to be the median line between the exclusive economic zones of the two countries.
The development was first commenced by China. But due to Japan's opposition, the two countries struck a deal in 2008 that Japanese companies would invest in the project.
Japan and China are seeking to sign a treaty over joint gas field development in the East China Sea. But talks over the treaty have been stalled since China unilaterally postponed them last year in protest at Japan's handling of ship collisions near a chain of Japanese islands, claimed by Beijing.
Chinese Foreign Minister Yang Jiechi told a news conference this week that the two countries must first ''''foster sound conditions and environment,'' in reference to the deadlocked negotiations.
The Asahi Shimbun reported Wednesday from Beijing quoting the state-owned company's official as saying that China has started drilling in the gas field.