Iraq told the United Nations the powder was destined for a pharmaceutical company that a former weapons inspector says was ordered by President Saddam Hussein before the 1991 Persian Gulf War to work on chemical and biological weapons. The powder, sold under the brand name Aerosil, has particles so small that, when coated with deadly poisons, can pass through the tiniest gaps in protective suits.

Experts inside and outside the U.S. government say they are not certain Iraq has dusty chemical weapons. Declassified U.S. intelligence documents say Iraq produced a dusty form of the blister agent mustard in the 1980s and used it during its eight-year war with Iran.

If Iraq made and used a powdered form of its deadliest nerve agent, VX, it could kill U.S. troops dressed in full protective gear, according to a 1990 Defense Intelligence Agency assessment. Although the military's protective suits have been improved since then, experts say dusty weapons could penetrate the new suits.

Pentagon officials refused to discuss the permeability of the new suits or whether Iraq has weapons that could pass through them. Such information is classified, they said.

The 1990 DIA document said soldiers could protect themselves by throwing rain ponchos over their chemical suits, which would reduce the fatality risk to near zero. One expert wrote later: "One gets the sense that this was recommended in the face of few other options."

The researcher, Eric Croddy of the private Center for Nonproliferation Studies, said dusty VX would be a serious danger to U.S. troops. VX is so toxic that, in its liquid form, a drop on the skin can kill within minutes.

"The effects of dusty VX, depending on how it gets in the body, would be somewhat faster," Croddy said. "It's certainly much more injurious and much more of a severe threat."

Dusty chemical weapons are formed by mixing a liquid chemical agent with a fine powder to coat the powder's tiny particles with the deadly poison. The particles' small size allows them to pass through the fabric of a protective suit and any tiny gaps around the seal of a gas mask.

The latest U.S. military protective suits have a layer of charcoal in the fabric to trap any poisons that might penetrate the outer covering, but particles small enough could pass through even the charcoal layer.

"The closest analogy is, no matter what happens when you go to the beach, you still get sand in your shorts," Croddy said.

The poisonous powder also would settle in the tiniest nooks and crannies of buildings and equipment, making decontamination extremely difficult. VX in its liquid form already is a decontamination challenge; the sticky poison is persistent and cannot be neutralized easily with substances such as bleach.

Even if dusty chemical weapons caused no U.S. casualties, they could force American soldiers to work in clumsy protective gear, decontaminate their equipment and avoid contaminated areas, giving Iraqi soldiers time to mount defences.

U.S. intelligence reports before the Gulf War said Iraq was capable of making dusty VX. They said that during the 1980s, Iraq imported more than 100 metric tonnes of Aerosil, a brand of fumed silicon dioxide.

The reports said no evidence was found that Iraq had made dusty VX, and UN inspectors were unable to find any hard evidence of that.

In September, the New York Times quoted an Iraqi defector as saying Saddam's chemical weapons scientists secretly began producing dusty VX as early as 1994.

Aerosil, made by the German chemical company Degussa AG, has an exceptionally small particle size: 12 nanometres. That means more than 2,100 of the particles strung together would be as thick as a human hair.

UN documents show that Iraq's Samarra Drugs Industry sought 25 metric tonnes of Aerosil last year under the UN-run oil-for-food program, and at least some of that order was delivered last month.

American intelligence agencies were not overly worried about the shipment of Aerosil because the substance has many legitimate uses.

Richard Spertzel, a former chief UN weapons inspector in Iraq, was stunned when a reporter told him about the shipment. Saddam ordered the Samarra enterprise to work on chemical and biological weapons in 1989, and his government still controls the company, Spertzel said.

"Do you know how much (dusty agent) a kilogram of that stuff makes? A couple cubic feet," Spertzel said. "This gives me another thing to worry about."

Hasmik Egian, a spokeswoman for the UN oil-for-food program, confirmed that Iraq received a shipment of colloidal silicon dioxide in October. Egian would not identify the brand name, source or amount of the silicon dioxide delivered.

The sale was held up for three weeks by the UN commission that oversees the oil-for-food program, Egian said. That commission, whose members include the United States, decided colloidal silicon dioxide was not a banned substance and allowed the transaction, Egian said.

A newly created UN body overseeing the oil-for-food program is considering Iraq's request to import more colloidal silicon dioxide, Egian said.