The U.S. State Department refrained Wednesday from commenting in detail on a fresh dispute between Japan and China over their agreed joint project of exploring gas fields in the East China Sea.

"We're not going to take a position on competing claims," the department's spokesman, John Kirby, told reporters, while saying in general terms any activities that could destabilize the region should be suspended.

"We want competing claims to be worked out peacefully and diplomatically," Kirby said.

Japan has criticized China for unilaterally building 12 new gas rig-like structures since June 2013 in the waters despite a 2008 bilateral agreement to develop gas fields through consultation, releasing photos.

The structures are within the Chinese side of a line the Japanese government draws midway between the coastlines of the two countries. The gap is too narrow to designate 200-nautical-mile exclusive economic zones for either side.

Japan and China have differences over China's claim to the Japan-administered Senkaku Islands in another area of the East China Sea. Washington has said it does not take a side on sovereignty but that the U.S. military would defend the Senkakus if necessary in line with a security treaty with Japan.