By Chris Hogg
BBC News, Tokyo
Japan has asked China to clarify reports that Beijing has imposed a ban on shipping near disputed gas fields in the East China Sea.
The gas dispute is one several irritants between the two sides
Both Japan and China have claimed rights to develop the Pinghu gas field which straddles an area both countries say is part of their territory.
News reports say a ban has been imposed while Chinese workers lay pipelines and cables in the area.
Japan's chief Cabinet secretary says Tokyo expressed its concern to Beijing.
This is the latest salvo in a long running dispute between Japan and China over oil and gas resources in the East China Sea.
China and Japan's exclusive economic zones (EEZs) overlap
Japan claims EEZ extends 200 nautical miles from its shore, while China claims EEZ extends to edge of its continental shelf
Two countries have never agreed a maritime border
The UN says it will arbitrate by May 2009
Also dispute ownership of Senkaku/Diaoyu islands
Under the United Nations Convention on the Law of the Sea, countries can claim economic zones extending 370km (230 miles) off their coast.
The disputed reserves lie in an area which falls into both countries' economic zones. Several rounds of negotiations have been held to try to resolve the issue without success.
Reports over the last few days have suggested the Chinese authorities have now banned all unauthorised shipping traffic around the Pinghu gas field until the end of September.
The suspicion is that China plans to expand its piping facilities during that period.
Japan's Chief Cabinet Secretary, Shinzo Abe, says that if the reports are true, the ban may breach the Law of the Sea. Bejing, he said, had given Tokyo assurances that it would look into the matter.
Japan's prime minister has assured reporters that the country will respond calmly to this latest development, an indication perhaps of just how damaging this dispute could be to relations between the two countries - which are already in poor shape.