Nearly 400 tonnes of explosives are missing from a site near Baghdad that was part of Saddam Hussein's dismantled atom bomb programme but was never secured by the US military, the United Nations said today.
The head of the UN's nuclear watchdog, Dr Mohamed ElBaradei, will immediately report the matter to the UN Security Council, a spokeswoman for the agency said.
The missing explosives could potentially be used to detonate a nuclear weapon or in conventional weapons, the agency said.
"ElBaradei has decided to inform the Security Council today," spokeswoman Ms Melissa Fleming said.
The New York Times, which broke the story today, said US weapons experts feared the explosives could be used in bombing attacks against US or Iraqi forces, which have come under increasing fire ahead of Iraq's elections due in January.
The UN's International Atomic Energy Agency (IAEA) has been barred from most of Iraq since the war and has watched from afar as its former nuclear sites have been systematically stripped by looters.
Ms Fleming said Dr ElBaradei informed Washington of the seriousness of the matter on October 15th after learning about the disappearance of the explosives on October 10th.
One substance found in large quantities at the Al Qaqaa facility was the explosive HMX, which Ms Fleming said had "a potential use in a nuclear explosive device as a detonator".
Prior to the March 2003 invasion of Iraq, the HMX had been sealed and tagged with the IAEA emblem while being stored at Al Qaqaa.
Iraq was permitted to keep some of its explosives for mining purposes after the IAEA completed its dismantling of Saddam's covert nuclear weapons programme after the 1991 Gulf war.
Diplomats at the IAEA have warned that materials useable in nuclear weapons could easily be shipped out of Iraq and sold to countries like Iran or terrorist groups believed to be interested in acquiring nuclear weapons.
The New York Times report cited White House and Pentagon officials - as well as at least one Iraqi minister - as acknowledging that the explosives vanished from the site shortly after the US-led invasion amid widespread looting.