Japan has lodged a protest with China over new exploration activities at a gas field in a contested area of the East China Sea, the government said Monday.
A Chinese drillship started operations, believed to be the sinking of a test borehole aimed at developing resources, near a Tokyo-proposed median line separating the countries' exclusive economic zones in mid-November, Japanese officials said.
"It is extremely regrettable that China has continued its unilateral development activities in the waters, while the boundary (between Japan and China) has not been fixed," Chief Cabinet Secretary Yoshihide Suga said at a press conference.
The top government spokesman said Japan protested through diplomatic channels "immediately after" confirming China's latest activities, without specifying the date.
Tokyo has repeatedly urged Beijing to halt unilateral resource exploration in the area, given that negotiations based on a 2008 bilateral accord on joint gas development around the line have been suspended due to heightened tensions over the sovereignty of a small group of uninhabited islands controlled by Japan in the East China Sea.
The move comes at a time when bilateral ties have shown signs of improvement with Shinzo Abe making the first official trip to China by a Japanese prime minister in seven years in late October.
During their talks in Beijing, Abe and Chinese President Xi Jinping confirmed that they should make efforts to ensure the East China Sea is a "sea of peace, cooperation and friendship."
In their latest meeting on Friday on the sidelines of the Group of 20 summit in Argentina, Abe called for Xi to restart talks about joint gas development in the sea as soon as possible.