Nearly 350 tons of conventional explosives have vanished from a former military complex in Iraq, the UN says.
The International Atomic Energy Agency (IAEA) said the explosives vanished from the al-Qaqaa facility near Baghdad during looting after the invasion.
It added that the explosives could be used in powerful conventional weapons or to detonate nuclear devices.
An IAEA spokesman said the agency was expected to inform the UN Security Council of its concerns on Monday.
The explosives are thought to have been taken from the al-Qaqaa complex, 25 kilometres (16 miles) south of Baghdad, at some point after 9 April 2003.
The IAEA said the US-led coalition had been warned about the danger posed by the explosives on several occasions.
It says the coalition forces were specifically told to keep the material secured.
The IAEA spokesman said the Iraqi interim government had alerted the agency about the missing explosives on 10 October.
The coalition forces in Iraq were informed on 15 October through the US administration, he said.
Access denied
"Our main concern is that if the materials fall into the wrong hands they could be used to commit terrorist acts," the spokesman added.
He said the explosives had been kept under seal by the IAEA until the US-led invasion of Iraq, after which point it had not been allowed to access the site.
The stolen material included HMX and RDX - key components in plastic explosives, which have been widely used in car bombings in Iraq.
BBC defence and security correspondent David Bamford said the IAEA had valid grounds for concern because even a kilogram of these high-powered explosives was enough to level a building.
Nobody seems willing to take responsibility, he said, with the IAEA arguing the war forced it to suspend its monitoring activities and the US-led occupation force putting other priorities ahead of preventing looting.
The IAEA earlier this month raised concerns over the disappearance of nuclear equipment and materials from Iraq's main nuclear site, Tuwaitha.
Iraq's Interim Technology Minister Rashid Omar confirmed the explosives had disappeared in an interview with the New York Times.
The paper claims US National Security Adviser Condoleezza Rice was told about the missing explosives only in the past month.
It is not yet known whether US President George W Bush has been informed.
US weapons experts are concerned the explosives could be used in bomb attacks against US and Iraqi forces in the run-up to Iraq's planned elections in January.
The chemical and explosives complex at al-Qaqaa was repeatedly visited by weapons inspectors before the war.
http://news.bbc.co.uk/1/hi/world/mid...st/3950493.stm