Iraq's acting Human Rights minister, Narmin Othman, said last night that a team would be dispatched to Fallujah to try to ascertain conclusively whether civilians had been killed or injured by the incendiary weapon. The use of white phosphorus (WP) and other incendiary weapons such as napalm against civilians is prohibited.

The announcement came as John Reid, the Secretary of State for Defence, faced mounting calls for an inquiry into the use of WP by British forces as well as what Britain knew about its deployment by American troops. Mr Reid said that he would look into the matter.

The move by the Iraqi government and the growing concern at Westminster follows the Pentagon's confirmation to The Independent earlier this week that WP had been used during the battle of Fallujah last November and the presentation of persuasive evidence that civilians had been among the victims.

The fresh controversy over Fallujah, which has raged for a full 12 months, was initially sparked last week by a documentary by the Italian state broadcaster, RAI, which claimed there were numerous civilian casualties. A Pentagon spokesman said yesterday he would "not be surprised" if WP had been used by US forces elsewhere in Iraq.

Lt-Col Barry Venable said the incendiary shells were a regular part of the troops' munitions. "I would not rule out the possibility that it has been used in other locations." The Pentagon's admission of WP's use - it can burn a person down to the bone - has proved to be a huge embarrassment to some elements of the US government.

In a letter to this newspaper, the American ambassador to London, Robert Tuttle, claimed that US forces "do not use napalm or WP as weapons" .

Confronted with the Pentagon's admission, an embassy spokesperson said Mr Tuttle would not be commenting further and "all questions on WP" should be referred to the Pentagon. The US embassy in Rome had issued a similar denial.

The size or scale of the inquiry to be undertaken by the Iraqi government is unclear, and it is not known when its investigators will arrive in Fallujah. An official with the human rights ministry said that while it was also not known how long the inquiry would take, "the people of Fallujah will be fully consulted". The Pentagon says the use of incendiary weapons against military targets is not prohibited.

But the article two, protocol III of the 1980 UN Convention on Certain Weapons bans their use against civilians.

Perhaps of crucial importance to the Iraqi investigators, the treaty also restricts their use against military targets "inside a concentration of civilians except when such military objective is clearly separated from the concentration of civilians".

Mr Reid confirmed yesterday that British troops had used WP in Iraq, though he said the shells had only been used to make smoke to obscure troops movements, which experts say is their primary function.

"Neither it nor any other munitions are used against civilians. It is not a chemical weapon," he said. Speaking at a Nato training exercise in Germany, where he was visiting British troops bound for Afghanistan, Mr Reid said the US's use of WP was a "matter for the US".

However, last week Mr Reid indicated that he would raise the issues contained within the RAI documentary if presented with evidence.

But last night MPs were openly dismissive of Mr Reid's comments and called for an inquiry, saying they had previously been misled about the US's use of napalm in Iraq. The US had drawn a distinction between conventional napalm and updated Mk 77 firebombs, which experts say are virtually identical.

Mike Gapes, the Labour chairman of the Foreign Affairs Select Committee, said: "I think there is an issue here about whether the chemical weapons convention should be strengthened to include this particular substance because it is defined as an incendiary not a chemical weapon, therefore it is excluded from certain definitions."

Sir Menzies Campbell, the Liberal Democrat foreign affairs spokesman, said: " 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." So far, the fall-out in the US over the revelation has been minimal. But the former president Bill Clinton yesterday told students at the American University of Dubai that he did not agree with invasion of Iraq.

The battle of Fallujah, an insurgent stronghold, took place over two weeks last November. It led to the displacement of 300,000 people. Reports from refugee camps and from an Iraqi doctor who stayed in the city during the fighting suggest numerous civilians suffered burns and "melting skin" . Photographs show rows of bodies charred almost beyond recognition.

Chemical legitimately used or a WMD?

What is white phosphorus?

White phosphorus is a highly flammable incendiary material which ignites when exposed to oxygen, and will burn human skin until all the oxygen is used up. A doctor from Fallujah described victims in the US siege "who had their skin melted".

White phosphorus, known as WP or Willy Pete in the military, flares in spectacular bursts with a yellow flame when fired from artillery shells and produces dense white smoke. It is used as a smokescreen for troop movements and to illuminate a battlefield.

Is it a chemical weapon?

No. White phosphorus has thermal properties which burn by heating everything around it, rather than chemical properties which attack the body's life systems . It therefore does not fall under the 1993 Chemical Weapons Convention. But protocol III of the 1980 Convention on Conventional Weapons bans its use as an incendiary weapon against civilian populations.

So what is all the fuss about?

The US ambassador to London, Robert Tuttle, said in a letter to The Independent that "US forces do not use napalm or phosphorus as a weapon. " The US position was that white phosphorus used as a smokescreen was legitimate - a position outlined by John Reid, the Defence Secretary, yesterday.

But a Pentagon statement on Tuesday appears to have shifted the argument. It said that US troops had used the white phosphorus as a weapon against insurgents. The State Department meanwhile corrected a statement, according to which white phosphorus was "fired into the air to illuminate enemy positions at night, not at enemy fighters". Now the argument focuses on whether those being targeted were insurgents or civilians, and, of course, in a place like Fallujah, this grey area gives the US more of a get-out clause.

Humanitarian law distinguishes between combatants and non-combatants. If the white phosphorus was used against insurgents they qualify as combatants and there has been no protocol breach.

Both the US and the UK have signed the convention, but Washington declared at the time of the signing of protocol III in 1995 that its military doctrine would abide by the protocol's provisions. These stipulate that the military distinguishes between military and civilian targets.

If it turns out that civilians were killed, what legal recourse is there?

If an Iraqi investigation provides evidence that civilians were killed by white phosphorus as a weapon, there is no recourse under the Conventional Weapons Convention.

However, the 1977 first protocol to the Geneva Conventions could be invoked. The United States has signed but not ratified the protocol which relates to the 4th Convention which considers the treatment of civilians.

Article 35 of the protocol makes it clear that the use and methods of use of "weapons of warfare are not unlimited." Any weapon or use of weapon that causes "superfluous or unnecessary suffering" is outlawed. The indiscriminate use of phosphorus on a civilian population would be covered.

Breaches of the Geneva Conventions are brought by individual countries and are usually heard by the United Nations at Security Council level, or in the International Court of Justice.

Peter Carter QC, an expert in international law and chairman of the Bar's human rights committee, said the latest US admissions raised serious concerns about whether white phosphorus was indiscriminately used against civilians. He called for an independent inquiry, possibly through the United Nations, into the use of white phosphorus in Iraq.

Why has all this come out so long after the Fallujah siege?

An Italian television documentary last week, accused the US of using white phosphorus in a "massive and indiscriminate way" against civilians at Fallujah.

This was denied by the Pentagon, but witnesses in the US military's Field Artillery magazine described firing '"shake and bake" missions at insurgents and high explosive shells to "take them out". The Independent's coverage of the RAI documentary and fallout prompted a letter from Ambassador Tuttle.

What does the US ambassador say now?

No comment. He referred all questions to the Pentagon.

Anne Penketh and Robert Verkaik

BUSH'S ARSENAL

The allegation

Napalm/Mark 77s

Widespread reports during the initial US-led invasion in March 2003 suggested marines had dropped incendiary bombs over the Tigris river and the Saddam canal on the way to Baghdad.

Cluster bombs

33 civilians, including many children, were reportedly killed in a US cluster bomb attack on Hilla, south of Baghdad. Reports of attacks on Basra were also widespread.

White Phosphorus

Coalition troops were reported to have used WP indiscriminately against civilians and insurgents during the Fallujah offensive of November 2004.

What the US said

Napalm/Mark 77s

The Pentagon denied reports it had used napalm, saying it had last used the weapon in 1993 and destroyed its last batch in 2001. "We don't even have that in our arsenal."

Cluster bombs

General Richard Myers, head of the Joint Chiefs of Staff, said coalition forces dropped nearly 1,500 cluster bombs during the war and only 26 fell within 1,500ft of civilian areas.

White Phosphorus

"[WP was used] very sparingly in Fallujah, for illumination. They were fired into the air to illuminate enemy positions at night, not at enemy fighters." US State Department

How the UK backed them up

Napalm/Mark 77s

"The US have confirmed to us they have not used Mk 77 firebombs, essentially napalm canisters, in Iraq at any time." Adam Ingram, Armed Forces minister, January 2004

Cluster bombs

The MoD said it supported the use of cluster bombs against legitimate military targets to protect British troops and civilians, insisting care was taken to avoid populated areas.

White Phosphorus

"Use of phosphorus by the US is a matter for the US," Tony Blair's spokesman said yesterday.

How the US came clean

Napalm/Mark 77s

It took five months for the US to admit its marines had used Mk 77 firebombs (a close relative of napalm) in the invasion. The Pentagon said their functions were "remarkably similar".

Cluster bombs

General Myers admitted: "In some cases, we hit those targets knowing there would be a chance of collateral damage." It was "unfortunate" that "we had to make these choices".

White Phosphorus

Pentagon spokesman Lt-Col Barry Venable said this week that WP had been used, "to fire at the enemy" in Iraq. "It burns... it's an incendiary weapon. That is what it does."

How the UK came clean

Napalm/Mark 77s

"First of all they didn't use napalm. They used a firebomb. It doesn't stick to your skin like napalm, it doesn't have the horrible effects of that. " John Reid, Defence Secretary

Cluster bombs

Adam Ingram, Armed Forces minister, said: "There were troops [and] equipment in and around built-up areas, therefore bombs were used to take out the threat to our troops."

White Phosphorus

The Government maintains it used WP in Iraq only to lay smoke screens. " We do not use white phosphorus against civilians," the Defence Secretary John Reid said.