In a report, the Government Accountability Office said US forces ultimately collected some 1,400 radiological sources used in medicine, industry and research, and removed 1,000 of them.

"According to DoD (Department of Defense) and State officials, however, the total number of unsecured radiological sources in Iraq remains unknown," the report said.

"For instance, even after DTRA (Defense Threat Reduction Agency) completed collecting and securing sources, according to Department of State officials, a neighboring country twice detected trucks leaving the country with unsecured radiological sources," the report said.

The fate of the radiological materials is a matter of concern because of the threat to civilians of radiation exposure and their possible use by terrorist to make "dirty bombs," the report said.

Radiological materials were found at many sites across Iraq by US forces, prompting security concerns by the International Atomic Energy Agency and some members of the US Congress.

But Defense Threat Reduction Agency officials said that attention did not shift to the collection of radiological materials until after it became clear that the broader hunt for weapons of mass destruction was unsuccessful.

"DoD was not ready to collect and secure radiological sources before the war began in March 2003 and about six months thearafter," the report said.

The department did not issue guidelines on how to collect and secure the materials until July 2003 and did not finalize a contract to do the work until September of that year, it said.

"Until radiological sources could be collected, some sources were looted and scattered, and some troops were diverted from combat duties to guard sources in diverse places," the report said.

Military commanders had no guidance or equipment for handling radiological materials in the early months of the war. So they took ad hoc measures to secure known sites at former Iraqi nuclear facilities, the report said.

In one instance, the military transported radiological material to the Tuwaitha Nuclear Research Center using an ice chest lined with lead bricks and a metal sheet to shield the driver of the vehicle from radiation.

After that, the military's nuclear disablement teams decided it was too risky to move radiological materials. Instead, coalition soldiers were detailed to guard sites, putting them at greater risk of attack.