Four Japanese 'filmed Chinese military zone'
Four Japanese are being investigated in China on suspicion of illegally filming in a military area, state media report.
It is the latest incident in a heated diplomatic row between the two nations.
Xinhua news agency said security officials in Shijiazhuang in Hebei province were taking action against the four after receiving a report about illegal activities.
Japan's foreign ministry confirmed that four people were being investigated but did not give details.
Kyodo news agency in Japan reported that the four involved were in the construction industry and were believed to have been preparing to bid on a project to dispose of chemical weapons abandoned by Japanese forces in China at the end of World War II.
The BBC's Roland Buerk in Tokyo says the detentions could further sour relations between Japan and China.
Tensions have been high since Japan detained a Chinese captain whose fishing boat collided with Japanese patrol boats earlier this month.
The trawler was sailing in an area claimed by both countries, near uninhabited islands which may have oil and gas deposits.
Japanese prosecutors suspect the captain may have rammed the patrol ships deliberately, and a court has given them until next Wednesday to decide whether or not to charge him.
Chinese Premier Wen Jiabao said afterwards that Japan bore full responsibility for the situation, and demanded the immediate release of the captain.
A spokesman for the foreign ministry in Tokyo, Hidenobu Sobashima, said Japan's embassy in Beijing had received confirmation four Japanese were being held but did not want to speculate on whether it was linked to the detention of the Chinese boat captain.
The dispute was brought up by Japanese Foreign Minister Seiji Maehara during a meeting on Thursday with US Secretary of State Hillary Clinton on the sidelines of the UN General Assembly in New York.
A US State Department spokesman said Mrs Clinton had urged both sides to settle the issue quickly before it had a long term impact on the region.