The Pentagon has admitted US forces used white phosphorus as "an incendiary weapon" during the assault last year on Fallujah.
A Pentagon spokesman's comments last night appeared to contradict the US ambassador to London who said that American forces did not use white phosphorus as a weapon.
Pentagon spokesman Lieutenant Colonel Barry Venable said that white phosphorus - which is normally used to lay smokescreens - was not covered by international conventions on chemical weapons.
But Professor Paul Rodgers of the University of Bradford department of peace studies said it probably would fall into the category of chemical weapons if it was used directly against people.
A recent documentary by the Italian state broadcaster, RAI, claimed that Iraqi civilians, including women and children, had died of burns caused by white phosphorus during the assault on Fallujah.
The report has been strenuously denied by the US, however Col Venable disclosed that it had been used to dislodge enemy fighters from entrenched positions in the city.
"White phosphorus is a conventional munition. It is not a chemical weapon. They are not outlawed or illegal," he said on the BBC Radio 4 PM programme.
"We use them primarily as obscurants, for smokescreens or target marking in some cases. However it is an incendiary weapon and may be used against enemy combatants."
Asked directly if it was used as an offensive weapon during the siege of Fallujah, he replied: "Yes, it was used as an incendiary weapon against enemy combatants".
He added: "When you have enemy forces that are in covered positions that your high explosive artillery rounds are not having an impact on and you wish to get them out of those positions, one technique is to fire a white phosphorus round into the position because the combined effects of the fire and smoke - and in some case the terror brought about the explosion on the ground - will drive them out of the holes so that you can kill them with high explosives," he said.
However in a letter yesterday to The Independent, the US ambassador to London, Robert Tuttle, denied that white phosphorus was deployed as a weapon.
"US forces participating in Operation Iraqi Freedom continue to use appropriate lawful conventional weapons against legitimate targets," he said.
"US forces do not use napalm or white phosphorus as weapons."
Col Venable said that a similar denial on the US State Department's website had been entered more than a year ago and was based on "poor information ".
Prof Rodgers said white phosphorus would be considered as a chemical weapon under international conventions if it was "deliberately aimed at people to have a chemical effect".
He told PM: "It is not counted under the chemical weapons convention in its normal use but, although it is a matter of legal niceties, it probably does fall into the category of chemical weapons if it is used for this kind of purpose directly against people."
Liberal Democrat foreign affairs spokesman Sir Menzies Campbell said later: "A vital part of the effort in Iraq is to win the battle for hearts and minds.
"The use of this weapon may technically have been legal, but its effects are such that it will hand a propaganda victory to the insurgency.
"The denial of use followed by the admission will simply convince the doubters that there was something to hide."
The Shadow Foreign Secretary Liam Fox said on today's BBC Radio 4 Today programme: "Clearly there needs to be more openness coming from the Pentagon but the claims at the moment are just claims.
"And I think that, although white phosphorus is a brutal weapon, we need to remember that we were talking about some pretty brutal insurgents. These were the people who were hacking off hostages' heads with knives."