Japan has lodged a strong protest with China over a unilateral natural gas exploration near contested waters.

Chief Cabinet Secretary Yoshihide Suga told a news conference that the Japanese government has confirmed that China is building a new facility producing natural gas near the median line between the two countries in the East China Sea.

Suga said China had not obtained consent from Tokyo to develop the area, which was formerly at the center of a joint project between two nations agreed on in June 2008.

He said Tokyo was “gravely” concerned over the developments.

“We cannot accept China developing natural gas resources unilaterally in waters where Japanese and Chinese claims overlap at a time when the boundary between the two countries has yet to be established in the East China Sea,” Suga said.

The two countries agreed in 2008 to jointly develop the Shirakaba natural gas field in the area straddling the median line. But China had pressed ahead with the development of the gas field, called Chunxiao in Chinese, drawing strong protests from Tokyo, reports The Asahi Shimbun.