Shoichi Nakagawa said at a press conference that, according to information obtained by Tokyo, the China National Offshore Oil Corporation (CNOOC), one of the nation's largest state-controlled hydrocarbon companies, had begun recovering natural gas from the Chunxiao deposit, which Japan says lies in one of its own exclusive economic zones.
Earlier this year, Tokyo accused Chinese companies of prospecting for hydrocarbon reserves in the field. Beijing responded by inviting Japan to cooperate on Chunxiao development projects, but had the negotiations suspended in May.
The privately-owned Japanese company Teikoku Oil, granted a license for production in the East China Sea in August this year, is only just beginning exploratory drilling in the area.
Beijing contests the legitimacy of Japan's border demarcation in the East China Sea.
Japanese authorities, including the National Defense Department, have repeatedly warned Beijing that its development of hydrocarbon deposits in disputed areas will complicate bilateral relations.