Japan said on Wednesday it has raised with China its serious concerns about the construction of a drilling rig near a disputed and potentially rich gas field 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 of the median line between the two nations, Chief Cabinet Secretary Yoshihiko Suga told a news briefing.

Japan considers the median line to be the appropriate border while China doesn't.

The two countries have agreed to jointly to develop the field, which lies in an area where both countries have overlapping claims to exclusive economic zones. Japan 's worry is that China would siphon gas from the deposit that stretches into what Japan considers its territory.

"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," Mr. 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 a specific response, Mr. Suga said.

The two countries agreed in 2008 that Japanese energy companies would be allowed to invest in the gas field 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 exclusive economic zones. But China says 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, was the most serious.

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