Reuters said it was "concerned and dismayed" by the detention of 36-year-old freelance Omar Abrahem al-Mashhadani and asked for an explanation for his treatment.

The group's global managing editor, David Schlesinger, demanded access to Mashhadani, who was arrested on August 8.

"We are very concerned and dismayed by this unexplained and prolonged detention of a journalist working for us and urge the US military either to release him or provide a full account of the accusations against him," Mr Schlesinger said.

"Ali al-Mashhadani is a professional journalist whose work for us brings him into contact with parties in conflict.

"In the interests of the independent media the United States has pledged to foster in Iraq, it is imperative the authorities act speedily to clarify with news organisations any suspicions arising from the activities of journalists working for us."

US military spokesmen have not said why they have detained Mashhadani, who has been working for Reuters as a cameraman and photographer for a year in Ramadi, the capital of Iraq's Anbar region.

Lieutenant Colonel Guy Rudisill, a spokesman for US detainee operations in Iraq, said the journalist was being held in Baghdad's Abu Ghraib prison and was not allowed to have visitors for the next 60 days.

Mashhadani's family have said he was detained after US marines found images on his cameras and his desktop computer.

Reuters said it had provided the US military with published work by Mashhadani showing scenes of conflict and gunmen operating in plain view of civilians.

This is not the first time Reuters has clashed with the US military over the treatment of its journalists.

Last year, three Iraqis working for Reuters were arrested at the scene of a helicopter crash near Falluja.

They claimed they were later sexually and physically abused by US soldiers for three days before being released, though a report cleared the troops involved.

Mashhadani's predecessor in Ramadi, Dhia Najim, was killed during fighting between US marines and insurgents in November last year.

Reuters said the exact circumstances of his death had not been clarified despite its requests.

The agency has had two cameramen killed by US troops in Iraq since the war began there in March 2003: a Ukrainian, Taras Protsyuk, and a Palestinian, Mazen Dana.