Japan has lodged a protest with China after detecting construction work for a new structure near the two countries' median line in the East China Sea.

Japan and China agreed in 2008 to jointly develop gas fields in the region. But as negotiations for a related treaty stalled, China has pursued its own development activities in the area, including construction near the median line.

Japan's foreign ministry announced on Friday that it confirmed China had started work on a new structure on the Chinese side of the line.

The head of the Japanese Foreign Ministry's Asian and Oceanian Affairs Bureau, Funakoshi Takehiro, lodged a protest with the Chinese Embassy in Tokyo.

He says it is "extremely regrettable" that China is unilaterally going ahead with the gas field development.

He urged China to swiftly resume talks for the related treaty around the 2008 agreement.

The ministry says this is the 17th Chinese structure of its kind.