International inspectors warned the United Nations Security Council on Monday that about 350 tonnes of explosives were missing from a former Iraqi military facility that had been monitored by the UN before the US invasion.
Mohamed ElBaradei, head of the International Atomic Energy Agency (IAEA), reported the disappearance to the UN, having learned about the missing material from the Iraqi government on October 10.
Senator John Kerry, the Democratic challenger to George W. Bush, seized on the news as further evidence of what he called the president's failure to deliver as commander-in-chief.
"This is 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 said.
The Pentagon said there were "any number of possibilities" of what might have happened at the Al-Qaqaa facility south of Baghdad, including that US forces had removed the explosives but not told the IAEA.
The explosives could be used for military or civilian purposes, but also for triggering a nuclear explosion.
Melissa Fleming, an IAEA spokeswoman, said the Iraqi government had said the explosives were stolen because of a lack of security.
Controversy has been focused on the management of Iraqi weapons facilities since the publication earlier this month of a letter by Mr ElBaradei to the UN Security Council outlining concern "about the widespread and apparently systematic dismantlement that has taken place at sites previously relevant to Iraq's nuclear programme."