Japan said on Wednesday it has voiced "serious concern" to China at the construction of a drilling rig near a disputed and potentially rich gasfield in the East China Sea.

A Chinese vessel has been seen building what appears to be a drilling platform about 26 kilometres (16 miles) west -- the Chinese side -- of the median line between the two nations, Chief Cabinet Secretary Yoshihiko Suga told a briefing.

Japan consider the median line to be the appropriate dividing line while China does not.

Tokyo has long suspected that Beijing would violate their agreement jointly to develop the field, which lies in an area where both countries' claimed exclusive economic zones (EEZ) overlap.

Japan's major concern was that China would siphon gas from the deposit that stretches into what Japan considers its own side.

"Our position remains that we cannot accept China's unilateral development in this region where Japan's and China's claims overlap, while delimitation in East China Sea remains undefined," Suga said.

"We have conveyed to China our serious concern about the activities of the crane vessel. We told them, through diplomatic channels, we cannot accept it," he said.

China has not given any specific response, Suga said.

The two countries agreed in 2008 that Japanese energy companies would be allowed to invest in the gasfield development, after China began working in the area's waters.

However, progress on cooperation soon stalled and Japan reported unilateral operations by Chinese ships in the area.

Japan has long pushed for the use of the median line between the two nations as the extent of their EEZs.

But China insists the border should be drawn closer to Japan, taking into account the continental shelf and other features of the ocean between them.

Relations have soured considerably over the last year, particularly over territorial and economic claims in the East China Sea. A dispute over the Tokyo-controlled Senkaku islands, which China calls the Diaoyus, is the most serious.