Japanese Prime Minister Fumio Kishida said on Saturday he was disappointed in China's actions, describing them as "unacceptable."

Speaking to reporters in the western city of Kyoto, he said the government had lodged a complaint against China via diplomatic channels.

The Japanese foreign ministry released a statement on Friday confirming an increase in Chinese efforts to develop natural resources in the East China Sea, including areas that are west of the midpoint between Japan and China.

One source of lasting tension between the world's second- and third-largest economies is a dispute over tiny islands in the East China Sea, which Japan controls but China also claims, and the waters around them.

Australian Associated Press