Some 20 percent of the known radioactive materials stored at Iraq's largest nuclear facility are unaccounted for, and U.S. nuclear experts have found radioactive patches on the ground where looters dumped out barrels believed to contain hazardous materials.

However, a senior commander said the great majority of the dangerous waste at the Tuwaitha (searchnuclear complex were still secure and were not leaking radiation.

"Eighty percent of the barrels are where they were before," said Col. Tim Madere (search), a specialist in unconventional weapons for the U.S. Army's V Corps (search).

It was unclear how many barrels were missing. The barrels had been previously catalogued and sealed by international arms inspectors.

The dormant Tuwaitha plant, once considered the heart of Saddam Hussein's nuclear program, has been repeatedly trashed by scavengers. It hasn't been operational for years. The Iraqis had been using it to store declared nuclear materials that were prohibited and sealed by the U.N. nuclear agency.

While the sprawling complex was considered one of the top sites where evidence of weapons of mass destruction might be found, it was left unguarded for days during the war. By the time weapons teams showed up to inspect the facility, so much had been destroyed that it was impossible to know what was missing.

A nuclear assessment team, which began a survey of the site Monday, has managed to track what remains of materials that had been monitored by the International Atomic Energy Agency.

Madere said several areas that had been closed-off by the IAEA "remain intact and sealed from the inside." U.S. troops didn't break the seals to enter those areas inside the facility, he said.

"We found no radiation outside except for in two small spots where some materials were probably dumped," Madere said in an interview at V Corps headquarters, located at the Baghdad international airport.

Before U.S. troops began guarding the entrance to the facility, reporters saw villagers removing storage barrels and dumping out contents matching the description of uranium oxide. They filled the barrels with drinking water, and some have since reported health problems.

Uranium, if enriched, is a key ingredient in nuclear weapons. The IAEA was monitoring 2 tons of enriched uranium and several tons of natural and depleted uranium stored at Tuwaitha.

The IAEA has been sharply critical of the U.S. handling of the site. U.S. Defense Secretary Donald Rumsfeld said Tuesday he would be willing to let members of the IAEA back into Iraq to assist them at nuclear sites. He couldn't say when or how the IAEA teams might work there, but said their previous knowledge and expertise would be welcome.

It was the first time U.S. officials have said the IAEA would be able to return to Iraq and was likely to be seen in the arms control community as an acknowledgment that the Americans need help.

Military commanders involved in the weapons hunt have said privately for weeks that they were operating without the basic information at the IAEA's disposal.

Rumsfeld's comments came a day after IAEA chief Mohamed ElBaradei issued a third plea to the U.S. government to let his teams conduct assessments at the nuclear sites. The United States didn't respond to his previous requests.

"We have a moral responsibility to establish the facts without delay and take urgent remedial action," ElBaradei said.

ElBaradei said he was deeply concerned about the "potential radiological safety and security implications of nuclear and radiological materials that may no longer be under control."

As part of the war, the United States chose to conduct its own weapons search rather than include the IAEA and U.N. inspectors, who found no evidence prior to the war to support the Bush administration's claims that Iraq had chemical and biological weapons and was reviving its nuclear program.

Madere, who has been working closely with the five different U.S. military teams that have taken part in the weapons search, said they haven't found any conclusive evidence of a program.

But some finds, he said, have heightened their suspicions.

"We found a chalkboard in Tuwaitha with diagrams that looked like they were planning to build a bomb," he said.

The teams are already conducting air monitoring around the area, and Madere acknowledged "there is still a potential health hazard."

Iraq has about 1,000 sites where radioactive materials are used in industry or medicine, but Tuwaitha, where Iraqis worked on the final design of a nuclear bomb before the 1991 Gulf War, has drawn the most concern.