IAEA Director Muhammad el-Baradei sent the Security Council a letter yesterday alerting it to a message from the Iraqi Ministry of Science and Technology.
The ministry reported the theft of more than 340 metric tons of highly explosive material -- known as HMX, RDX, and PETN. HMX is powerful enough to ignite the fissile material in an atomic bomb and set off a nuclear chain reaction. HMX and RDX are also key components in powerful plastic explosives such as C-4 and Semtex. The ministry's message to the IAEA said the material was lost after 9 April 2003, "due to lack of security."
The material was sealed and tagged by the IAEA at the Al-Qaqaa military facility prior to the U.S.-led invasion of Iraq in March 2003. Agency spokeswoman Melissa Fleming told reporters that the whereabouts of the material is unknown.
"This isn't the first time that the IAEA has reported that material or equipment under IAEA watch had been looted or gone missing. In fact, just two weeks ago, Dr. el-Baradei reported to the Security Council that on many sites we had observed whole buildings being stripped completely and dismantled and the contents within having gone missing," Fleming said.
The first report to the Security Council was based on agency monitoring of Iraq mainly through satellite surveillance. U.S. officials said at the time they had taken measures to improve security, helping Iraqi officials put new controls in place prohibiting the export of items related to weapons of mass destruction.
The latest report was made public by the agency following an article about the missing explosives in yesterday's "New York Times," citing Iraqi, U.S., and UN officials. Bush administration officials yesterday stressed that no nuclear material was involved, but said they are treating the report seriously.
U.S. State Department spokesman Adam Ereli said it has been a challenge to safeguard Iraq's many weapons sites.
"We, from the very beginning of Operation Iraqi Freedom, did everything we could to secure arms caches throughout the country. But given the number of arms and the number of caches and the extent of militarization of Iraq, it was impossible to provide 100 percent security for 100 percent of the sites," Ereli said.
But weapons experts say the explosive material stored at Al-Qaqaa was widely known to be part of Iraq's nuclear program.
Gary Milhollin directs the Wisconsin Project on Nuclear Arms Control, a U.S.-based foundation that works to prevent the proliferation of weapons of mass destruction.
"That was [the Iraqis'] premier site for designing implosion devices, and they were certainly using these materials to do that. And that's where the experts were. So that was a site that everybody knew about and that should have been a high priority for securing," Milhollin said.
David Kay, the former chief U.S. weapons inspector in Iraq, said yesterday that the ongoing Iraqi insurgency is being "fueled" by such incidents.
"The insurgency has been fueled by Iraqi explosives that were there and left unguarded at the end of the war and that the insurgents took [away]," Kay said.
In el-Baradei's first report to the Security Council this month, he reminded states of their obligation to inform the IAEA about changes at sensitive sites under agency review. The letter to the IAEA by Muhammad Abbas of Iraq's Ministry of Science and Technology said: "We feel an urgent updating of the registered materials is required."
The report of the missing explosives comes just one week ahead of the U.S. presidential election. The main challenger to U.S. President George W. Bush is Democrat John Kerry, who used the report to portray the administration as dangerously unprepared to cope with the challenges in Iraq.
"In May of this year, the administration was warned that terrorists may be helping themselves to, quote -- this was the warning -- 'the greatest explosives bonanza in history.' And now we know that our country and our troops are less safe because this president failed to do the basics. This is one of the great additional blunders of Iraq," Kerry said.
Bush did not address the issue during his campaign appearances yesterday.
The IAEA left Iraq ahead of the U.S.-led war to oust Iraqi leader Saddam Hussein and has returned for two limited visits. The UN body responsible for monitoring Iraq's other weapons of mass destruction efforts -- UNMOVIC -- has not returned to Iraq since the war, although it continues to conduct surveillance of the country.
The Security Council is due to decide on the future role of the two organizations in Iraq but has no immediate plans to do so.
(RFE/RL's Ryan Gallagher contributed to this report.)