Updated

The Iraqi interim government believes that more than 380 tons of dangerous explosives are missing from a military installation south of Baghdad, FOX News confirmed Monday.

International Atomic Energy Agency (search) chief Mohamed ElBaradei (search) is expected to report the materials' disappearance to the U.N. Security Council later Monday.

Iraqi officials sent a letter to the IAEA on Oct. 10 to inform the agency that tons of HMX and RDX explosives were missing from the Al Qaqaa (search) military installation south of Baghdad. Officials believe the material was looted following the fall of Saddam Hussein’s government in April 2003.

HMX and RDX are key ingredients in plastic explosives such as C-4 (search) and Semtex (search) — substances so powerful that Libyan terrorists needed just 1 pound to blow up Pan Am Flight 103 (search) over Lockerbie, Scotland, in 1988, killing 170 people.

IAEA officials fear the material can be made into a plastic explosive, which can be molded and used to detonate nuclear weapons. It can also be used as a conventional weapon and IAEA spokesman Mark Gwozdecky said the agency is “concerned” about the loss.

Deputy White House spokesman Trent Duffy told FOX News that the Defense Department is looking into what happened to the missing explosives — which Duffy characterized as conventional, saying there's nothing to suggest they're nuclear weapons.

Duffy told FOX that the IAEA didn't inform the U.S. government about the lost explosives until Oct. 15.

In Washington, Democratic presidential hopeful John Kerry's campaign said the Bush administration "must answer for what may be the most grave and catastrophic mistake in a tragic series of blunders in Iraq."

"How did they fail to secure ... tons of known, deadly explosives despite clear warnings from the International Atomic Energy Agency to do so?" senior Kerry adviser Joe Lockhart (search) said in a statement.

The Iraqis told the nuclear agency the materials had been stolen and looted because of a lack of security at governmental installations, IAEA spokesman Melissa Fleming said.

"We do not know what happened to the explosives or when they were looted," she told The Associated Press.

National Security Adviser Condoleezza Rice (search) reportedly was informed of the missing explosives in the past month.

"Upon receiving the declaration on October 10, we first took measures to authenticate it," Fleming said. "Then on October 15, we informed the multinational forces through the U.S. government with the request for it to take any appropriate action in cooperation with Iraq's interim government."

"Mr. ElBaradei wanted to give them some time to recover the explosives before reporting this loss to the Security Council, but since it's now out, ElBaradei plans to inform the Security Council today" in a letter to the council president, she said.

Before the war, inspectors with the Vienna-based IAEA had kept tabs on the so-called "dual use" explosives because they could have been used to detonate a nuclear weapon. Experts say HMX can be used to create a highly powerful explosion with enough intensity to ignite the fissile material in an atomic bomb and set off a nuclear chain reaction.

IAEA inspectors pulled out of Iraq just before the 2003 invasion and have not yet been able to return despite ElBaradei's repeated urging that the experts be allowed back in to finish their work.

ElBaradei told the U.N. Security Council before the war that Iraq's nuclear program was in disarray and that there was no evidence to suggest it had revived efforts to build atomic weaponry.

Al Qaqaa, a sprawling former military installation about 30 miles south of Baghdad, was placed under U.S. military control but repeatedly has been looted, raising troubling questions about whether the missing explosives have fallen into the hands of insurgents battling coalition forces.

Saddam was known to have used the site to make conventional warheads, and IAEA inspectors dismantled parts of his nuclear program there before the 1991 Gulf War. The experts also oversaw the destruction of Iraq's chemical and biological weapons.

The nuclear agency pulled out of Iraq in 1998, and by the time it returned in 2002, it confirmed that 35 tons of HMX that had been placed under IAEA seal were missing. HMX and RDX are the key components in plastic explosives, which insurgents have widely used in a series of bloody car bombings in Iraq.

"These explosives can be used to blow up airplanes, level buildings, attack our troops and detonate nuclear weapons," Lockhart said.

"The Bush administration knew where this stockpile was, but took no action to secure the site. They were urgently and specifically informed that terrorists could be helping themselves to the most dangerous explosives bonanza in history, but nothing was done to prevent it from happening," he said.

"This material was monitored and controlled by U.N. inspectors before the invasion of Iraq. Thanks to the stunning incompetence of the Bush administration, we now have no idea where it is," Lockhart said. He demanded the White House explain "why they failed to safeguard these explosives and keep them out of the hands of our enemies."

ElBaradei told the United Nations in February 2003 that Iraq had declared that "HMX previously under IAEA seal had been transferred for use in the production of industrial explosives, primarily to cement plants as a booster for explosives used in quarrying."

"However, given the nature of the use of high explosives, it may well be that the IAEA will be unable to reach a final conclusion on the end use of this material," ElBaradei warned at the time.

"A large quantity of these explosives were under IAEA seal because they do have a nuclear application," Fleming said Monday.

The nuclear agency has no concrete evidence to suggest the seals were broken, Fleming said, but a diplomat familiar with the agency's work in Iraq said the seals must have been broken if the explosives were stolen.

The Associated Press contributed to this report.