Ridding Iraq of banned weapons was the primary justification for the US-British invasion of Iraq, but evidence of chemical, biological or nuclear weapons has so far eluded forces on the ground.

Defense Secretary Donald Rumsfeld refused to comment on the reported discovery, noting that initial finds often do not pan out.

"We have to recognize that almost all first reports that we get turn out to be wrong. There tend to be changes in them. And as a result, we have to take our time, and look at it," he said.

But US defense officials, speaking on condition of anonymity, confirmed that barrels found by US troops at a military site near Karbala had registered positive for chemical warfare agents in field tests.

"Preliminary tests show they have chemical agents," said a defense official, who spoke on condition of anonymity.

A senior US defense official, who asked not to be identified, also said preliminary tests detected chemical warfare agents at the site but cautioned that the finding was not definitive.

An NBC News reporter embedded with US troops said a dozen barrels found at a site 30 miles (20 kilometers) from Karbala tested positive as blister and nerve agents.

A reporter for the Knight-Ridder newspaper chain said suspicious material was found at a compound near Hindiya, about 60 miles (35 miles) south of Baghdad, but initial tests were inconsistent.

A military intelligence officer for the US 101st Airborne Division's aviation brigade, Captain Adam Mastrianni, later told AFP that comprehensive tests determined the presence of pesticide compounds.

The senior defense official, however, said the tests that were positive for chemical warfare agents were "something recent."

"It's being looked at," he said. "It too early to make a definitive proclamation."

Defense officials here said they had no information about a separate report that Iraqi BM-21 rockets that appeared armed with chemical warheads were found at a warehouse near the US-occupied Saddam Hussein International Airport near Baghdad.

US fears that the Iraqi regime would use chemical or biological weapons against coalition forces has so far failed to materialize.

The coalition forces have mounted an intense psychological warfare campaign aimed at convincing Iraqi military officers not to carry out orders to use weapons of mass destruction, warning that they will be prosecuted for war crimes if they do.

US forces also have aggressively targeted Iraqi artillery and rocket systems capable of delivering chemical or biological warheads.

Iraq has insisted throughout the conflict that it no longer has any weapons of mass destruction, although it has acknowledged in the past producing tonnes of chemical weapons in the past, including sarin, cyclosarin, VX nerve agents and mustard gas.

Iraq used chemical agents against Iran in 1980s and against Kurdish villagers in northern Iraq in 1988.

The Iraqi commander who ordered the gas attacks on the Kurds, Ali Hassan al-Majid, was believed to have been killed Saturday in an air strike on his house in Basra, US and British officials said.

"We believe the reign of terror of Chemical Ali has come to an end," Rumsfeld told reporters.

SPACE.WIRE