The threat of an Iraqi chemical or biological attack against coalition forces has become "negligible", US military sources in Iraq told AFP on Thursday.

"Now that we have penetrated Baghdad's outer ring, the likelihood [of a chemical or biological attack] is negligible," said Captain Adam Mastrianni, the intelligence officer of the 101st Airborne Division's Aviation Brigade.

"The commanding general of the 101st, General David Petraeus gave the order at 9pm [4am AEST Friday] that soldiers in the division would be able to take off their anti-chemical and biological suits as of Friday morning," he added.

Mr Mastrianni's aviation brigade is still based near the central Iraqi town of Najaf. He explained to the AFP correspondent travelling with the US troops that the non-conventional threat against coalition troops engaged in Iraq had globally receded.

US military planners had feared that Iraqi President Saddam Hussein might launch a chemical attack when US and British troops reached three zones.

"We thought if he was going to use chemicals, the first major town he was going to use them in was in Karbala," a major Shiite town less than 100km south of Baghdad, Mr Mastrianni said.

"The next major zone, was when we [coalition troops] penetrated outer Baghdad," he said, explaining that could be defined as areas within 30km of the capital.

"Now the 3ID [3rd Division] is in Baghdad and no chemicals have been used," he added.