Several hundred tonnes of sophisticated high explosive have been stolen from a Saddam-era military facility in Iraq, despite repeated warnings from UN weapons watchdogs that US troops should secure the site.

Mohamed ElBaradei, Director-General of the International Atomic Energy Agency, will formally notify the UN Security Council about the theft later today, two weeks after being informed by Iraq's interim Government that about 350 tonnes of explosive had gone missing.

The explosives, though conventional, are considered to be "dual use" - meaning that they could be used to detonate a nuclear device.

"Our main concern is that if the materials fall into the wrong hands, they could be used to commit terrorist acts," an IAEA spokesman said.

News of the theft was seized on by Senator John Kerry, the Democratic presidential challenger, as evidence that the President Bush and his administration had rushed into the war in Iraq without proper planning.

"This is one of the great blunders of Iraq, one of the greatest blunders of this administration and the incredible incompetence of this President and this administration has put our troops at risk and this country at greater risk," Mr Kerry told a campaign rally in Dover, New Hampshire.

"The unbelievable blindness, stubbornness, arrogance of this administration to do the basics has now allowed this President to once again fail the test of being commander in chief."

The Bush campaign dismissed Mr Kerry's criticism, without responding to the allegations. "John Kerry has no vision for fighting and winning the War on Terror, so he is basing his attacks on the headlines he wakes up to each day," said a campaign spokesman, Steve Schmidt.

The IAEA said that it had received a declaration from the Iraqi Ministry of Science and Technology on October 10, informing them that 342 tonnes of HMX, RDX and PETN had been stolen from the vast al-Qaqaa weapons facility, 30 miles south of Baghdad, "due to lack of security".

HMX and RDX are the key components in plastic explosives, which insurgents have widely used in a series of bloody car bombings, mainly targeting Iraq's fledgeling security services.

The theft was revealed by The New York Times this morning, in a report from Baghdad which said that the facility was "supposed to be under American military control, but is now a no man's land, still picked over by looters as recently as Sunday".

IAEA inspectors had kept a close eye on the facility for many years, but have not been allowed into Iraq since the US-led invasion last year. It is not known when the explosives, which would fill several dozen trucks, went missing.

James Hider, The Times correspondent in Baghdad, said that it appears the explosives were looted in the period of lawlessness after last year's invasion.

Hider said that similar explosives have been used in car bombings in Baghdad and other cities, although most "improvised explosive devices" planted by roads are made from old munitions such as landmines and artillery shells.

He said: "I think the main worry is that this high-grade explosive will get into the hands of international terrorists - terrorist groups operating outside Iraq. People here are used to things being blown up and this stuff has been missing for a year, so presumably they would be using it already.

"It's very embarrassing for the Americans. The very thing the war was supposed to prevent, it has in fact facilitated."

The theft raised fears that Iraqi insurgents could use the explosives in major bombing attacks against US, Iraqi or British forces. The New York Times said that just a pound of a similar explosive was used to bring down Pan Am Flight 103 over Lockerbie in 1988.

News of the theft could prove a major embarrassment for Mr Bush, only eight days out from the presidential election. Opinion polls show the race to be virtually deadlocked, with one poll today giving Mr Bush a slender three-point lead.

Joe Lockhart, a Kerry aide, said in a statement: "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..

"This material was monitored and controlled by UN inspectors before the invasion of Iraq. Thanks to the stunning incompetence of the Bush administration, we now have no idea where it is."

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