Japan has lodged a protest with China over its gas field development in the East China Sea, after spotting flames at one of the Chinese-built structures in the waters.

The Japanese Foreign Ministry on Monday confirmed flames rising from a structure Beijing set up in May on China's side of an intermediate line between the two countries. Officials see those flames as a sign that natural gas production is underway.

This is part of the area that Tokyo and Beijing agreed to jointly develop as gas fields in 2008. A treaty on the joint development has yet to be signed, but China has been pushing ahead with development on its own.

China has installed a total of 18 structures in the region, with flames confirmed at 13 of them so far.

Funakoshi Takehiro, who heads the Foreign Ministry's Asian and Oceanian Affairs Bureau, lodged a protest with the Chinese Embassy in Tokyo.

Funakoshi called China's unilateral development "extremely regrettable," and demanded that Beijing return to the table to negotiate a treaty.