The Iraqi government this month reported the disappearance of 380 tons of HMX and RDX explosives to the International Atomic Energy Agency (IAEA), which had monitored the explosives before the war because they could be used as a trigger for nuclear devices.

"This is a first report. We do not know when -- if those weapons did exist at that facility -- they were last seen, and under whose control they were last in," Pentagon spokesman Larry DiRita said.

"It's very possible -- certainly it's plausible -- that it was the Saddam Hussein regime that last had control of these things," he told AFP.

DiRita said US forces visited the Al-Qaqaa site several times after the US invasion of Iraq as part of a US-led search for Iraq's weapons of mass destruction and related material.

But he said it is unclear whether the missing explosives were at the site during those visits.

"The forces searched 32 bunkers and 87 other buildings at the facility. Some explosive material was discovered, none of it carried IAEA seals. They did find stuff there. They probably secured it or destroyed it," he said.

DiRita said Iraq was swimming in weapons and ammunition after the war. More than 500 weapons sites were identified after the war, and some 200,000 tons of ammunition have been destroyed by US forces.

"I'm told they (US forces) made several visits to that facility looking for WMD related (material), and obviously we need to learn more about exactly what it is they saw there," he said.

"There have been these reports that there is evidence this place has been looted. But I think that's something to be very careful about. That place was not in anybody's control but Saddam Hussein's from the beginning of the war until sometime in April," he said.

"It's just really difficult to say with any kind of certainty what happened to those weapons, and who were the last people who had control of them. But I think it's at least arguable that the last person who had control of them was the Saddam Hussein regime," he said.