Photo/IllutrationAn offshore drilling platform that China built in 2015 in the East China Sea. (Captured from the Foreign Ministry website)

Japan has lodged a protest with China over the deployment of a drilling vessel to the East China Sea for maritime gas exploration, Chief Cabinet Secretary Yoshihide Suga told reporters on Dec. 3.

Suga said at a news conference that it was confirmed that China had sent a drillship to the Chinese side of the Japan-China median line between the two countries in the East China Sea. It was also confirmed that it drilled for gas there, which Japan protested through diplomatic channels.

According to the Foreign Ministry, the vessel was spotted in September in surrounding waters, which the ministry protested to China. In mid-November, it was discovered that the ship entered the waters again and drilled for gas.

“China’s unilateral activity can’t be accepted,” the ministry said, as no maritime border has been set at the site.

In 2008, Japan and China signed an agreement to jointly develop gas fields in the East China Sea.