The Pentagon's confirmation that it used white phosphorus as a weapon during last year's offensive in the Iraqi city of Falluja has sparked criticism.
The BBC News website looks at the facts behind the row.
What is white phosphorus?
White phosphorus is a solid, waxy man-made chemical which ignites spontaneously at about 30C and produces an intense heat, bright light and thick pillars of smoke.
The US military says it used white phosphorus to flush out insurgents
It continues to burn until deprived of oxygen and, if extinguished with water, can later reignite if the particles dry out and are exposed again to the air.
Also known by the military as WP or Willie Pete, white phosphorus is used in munitions, to mark enemy targets and to produce smoke for concealing troop movements.
It can also be used as an incendiary device to firebomb enemy positions.
What are its effects?
If particles of ignited white phosphorus land on a person's skin, they can continue to burn right through flesh to the bone. Toxic phosphoric acid can also be released into wounds, risking phosphorus poisoning.
Skin burns must be immersed in water or covered with wet cloths to prevent re-combustion until the particles can be removed.
Exposure to white phosphorus smoke in the air can also cause liver, kidney, heart, lung or bone damage and even death.
A former US soldier who served in Iraq says breathing in smoke close to a shell caused the throat and lungs to blister until the victim suffocated, with the phosphorus continuing to burn them from the inside.
Long-term exposure to lesser concentrations over several months or years may lead to a condition called "phossy jaw", where mouth wounds are caused that fail to heal and the jawbone eventually breaks down.
How did the US use it?
The US initially denied reports it had used white phosphorus as a weapon in Falluja in November 2004, saying it had been used only for illumination and laying smokescreens.
Spontaneously flammable chemical used for battlefield illumination
Contact with particles causes burning of skin and flesh
Use of incendiary weapons prohibited for attacking civilians (Protocol III of Convention on Certain Conventional Weapons)
Protocol III not signed by US
However, the Pentagon has now confirmed the substance was used as an "incendiary weapon" during the assault.
It was deployed as a conventional - rather than chemical - munition, the military said, and its principal use was as a smokescreen and to mark enemy targets.
However, the US has now admitted its forces also used white phosphorus rounds to a lesser extent to flush enemy forces out of covered positions, allowing them to be targeted with high explosives.
The US military denies using the chemical against civilians and stresses its deployment is not illegal.
What are the international conventions?
White phosphorus is covered by Protocol III of the 1980 Convention on Conventional Weapons, which prohibits its use as an incendiary weapon against civilian populations or in air attacks against enemy forces in civilian areas.
The US - unlike 80 other countries including the UK - is not a signatory to Protocol III.
How widely is it used?
White phosphorus was extensively used as a smokescreen by Russian forces in the battle for the Chechen city of Grozny in December 1994.
The UK has confirmed it has the chemical and has used it in Iraq - but only to lay smokescreens.
The use of white phosphorus in incendiary devices dates back to World War I and beyond.
It was used in World War II predominantly for smoke screens, marker shells, incendiaries, hand grenades and tracer bullets.
The chemical also has many non-military applications, being widely used by industry in products ranging from toothpaste to fertiliser.
What is the current furore about?
The row began when Italy's state television network Rai claimed that white phosphorus had been used against civilians in a "massive and indiscriminate way" during the Falluja offensive.
Its documentary, Falluja - The Hidden Massacre, alleged that Iraqi civilians, including women and children, had died of the burns it caused.
The allegations prompted demonstrations outside the US embassy in Rome by anti-war protesters and left-wing Italian politicians. Some European Parliament members have also demanded an inquiry into the munitions' use.
Critics say phosphorus bombs should not be used in areas where there is a risk they could cause serious burns or death to civilians.
Some have claimed the use of white phosphorus contravenes the 1993 Chemical Weapons Convention. This bans the use of any "toxic chemical" weapons which causes "death, harm or temporary incapacitation to humans or animals through their chemical action on life processes".
Professor Paul Rogers, of the University of Bradford's department of peace studies, told the BBC that white phosphorus could probably be considered a chemical weapon if deliberately aimed at civilians.
Washington's initial denial of the use of white phosphorus as a weapon against enemy forces and subsequent retraction have been seen as damaging to its public image - despite the fact it has breached no treaty obligations.