Military officers had said the soldiers, from the 101st Airborne Division's 1st Brigade, suffered nausea as well as the blisters after walking into a building that stored Iraqi ammunition on Monday afternoon.

"The division surgeon doesn't think it is mustard gas," 101st spokesman Major Hugh Cate told AFP.

"He thought it was a false alarm."

A 101st military intelligence officer, Captain Adam Mastrianni, had earlier said initial tests had shown the substance in the building was mustard gas, but more thorough tests were being conducted on Tuesday to confirm that.

"It's fairly likely it was in fact mustard gas," Mastrianni, attached to the division's Aviation Brigade, said.

A spokeswoman at US Central Command in Qatar, which is directing the US-British war on Iraq, had no immediate reaction to the mustard gas report.

"We have no confirmation of that," Major Randi Steffy said.

Another 101st spokesman, Lieutenant Eric Lake, said the five soldiers had been taken to a military hospital with blisters and nausea, but their conditions were not believed to be life-threatening.

Mastrianni said it was not yet clear where in the building the suspect substance was stored or whether it had been packed into artillery shells.

If the substance is confirmed as mustard gas, Mastrianni said this was a breach of Iraqi President Saddam Hussein's commitment not to produce or use chemical weapons and other weapons of mass destruction.

"It's still a chemical agent and it's not supposed to be utilised," he said.

"The bottom line is it's one of the items he was supposed to be destroy."

US forces "secured" Najaf, 100 kilometres (160 miles) southwest of Baghdad, last week after about a week of intense fighting with forces loyal to Saddam.

The 101st soldiers are still in the Shiite Muslim holy city to ensure it remains secure for military and humanitarian operations.

SPACE.WIRE