Allegations that civilians were victims of white phosphorus bombs in the Falluja battle last year has put the spotlight on a weapon that showers down incendiary particles that burn right through skin.
"A bullet goes through skin even faster than white phosphorus does," General Peter Pace, the chairman of the Joint Chiefs of Staff, said at a Pentagon press conference in Washington on Tuesday.
"So I would rather have the proper instrument applied at the proper time, as precisely as possible, to get the job done, in a way that kills as many of the bad guys as possible and does as little collateral damage as possible."
He said no military went to greater lengths to avoid civilian casualties than the US military.
Pace said white phosphorus was mainly used to mark targets for air strikes, and to create smoke screens for moving forces around a battlefield unseen.
"White phosphorus is a legitimate tool of the military," he said.
"It is not a chemical weapon. It is an incendiary. And it is well within the law of war to use those weapons as they're being used, for marking and for screening," Pace said.
An account of the battle for Falluja published in a US army journal in April said white phosphorus was also used to flush fighters out of trench lines in what were described as "shake and bake" missions.
In an unusual exchange, Pace said American troops in Iraq had a duty to intercede and stop abuse of prisoners by Iraqi security personnel, but Defence Secretary Donald Rumsfeld contradicted him, saying he believed the general meant to say the US soldiers had to report the abuse, not stop it.
Rumsfeld: Inhumane behaviour
is worrisome and harmful
Pace, chairman of the Joint Chiefs of Staff, stood firm. "If they are physically present when inhumane treatment is taking place, sir, they have an obligation to try to stop it," Pace told his civilian boss.
The exchange occurred on Tuesday during a discussion at the Pentagon news conference about the relationship between US forces in Iraq and an Iraqi government considered sovereign by the US.
A questioner asked whether, since the US and its allies train Iraqi forces, the foreigners might be responsible for preventing mistreatment of people under arrest.
"There are a lot of people involved in this, dozens of countries trying to help train these Iraqi forces. Any instance of inhumane behaviour is obviously worrisome and harmful to them when that occurs," Rumsfeld said.
"Iraq knows, of certain knowledge, that they need the support of the international community. And a good way to lose it is to make a practice of something that is inconsistent with the values of the international community," the US defence secretary said.
Rumsfeld added: "Now, you know, I can't go any further in talking about it. Obviously, the United States does not have a responsibility when a sovereign country engages in something that they disapprove of."
Pace, the highest-ranking American in uniform, was asked what orders the troops had to handle such incidents. He said: "It is absolutely the responsibility of every US service member if they see inhumane treatment being conducted, to intervene, to stop it."
Top US general Pace (L) joined
issue with Rumsfeld on Tuesday
He said soldiers who hear of but did not see an incident should deal with it through superiors of the offending Iraqis.
That was when Rumsfeld stepped to the microphone and said: "I don't think you mean they have an obligation to physically stop it. It's to report it."
Pace then repeated to Rumsfeld that intervening when witnessing abuse was the order, not reporting it.