American pilots dropped the controversial incendiary agent napalm on Iraqi troops during the advance on Baghdad. The attacks caused massive fireballs that obliterated several Iraqi positions.

The Pentagon denied using napalm at the time, but Marine pilots and their commanders have confirmed that they used an upgraded version of the weapon against dug-in positions. They said napalm, which has a distinctive smell, was used because of its psychological effect on an enemy.

A 1980 UN convention banned the use against civilian targets of napalm, a terrifying mixture of jet fuel and polystyrene that sticks to skin as it burns. The US, which did not sign the treaty, is one of the few countries that makes use of the weapon. It was employed notoriously against both civilian and military targets in the Vietnam war.

The upgraded weapon, which uses kerosene rather than petrol, was used in March and April, when dozens of napalm bombs were dropped near bridges over the Saddam Canal and the Tigris river, south of Baghdad.

"We napalmed both those [bridge] approaches," said Colonel James Alles, commander of Marine Air Group 11. "Unfortunately there were people there ... you could see them in the [cockpit] video. They were Iraqi soldiers. It's no great way to die. The generals love napalm. It has a big psychological effect."

A reporter from the Sydney Morning Herald who witnessed another napalm attack on 21 March on an Iraqi observation post at Safwan Hill, close to the Kuwaiti border, wrote the following day: "Safwan Hill went up in a huge fireball and the observation post was obliterated. 'I pity anyone who is in there,' a Marine sergeant said. 'We told them to surrender.'"

At the time, the Pentagon insisted the report was untrue. "We completed destruction of our last batch of napalm on 4 April, 2001," it said.

The revelation that napalm was used in the war against Iraq, while the Pentagon denied it, has outraged opponents of the war.

"Most of the world understands that napalm and incendiaries are a horrible, horrible weapon," said Robert Musil, director of the organisation Physicians for Social Responsibility. "It takes up an awful lot of medical resources. It creates horrible wounds." Mr Musil said denial of its use "fits a pattern of deception [by the US administration]".

The Pentagon said it had not tried to deceive. It drew a distinction between traditional napalm, first invented in 1942, and the weapons dropped in Iraq, which it calls Mark 77 firebombs. They weigh 510lbs, and consist of 44lbs of polystyrene-like gel and 63 gallons of jet fuel.

Officials said that if journalists had asked about the firebombs their use would have been confirmed. A spokesman admitted they were "remarkably similar" to napalm but said they caused less environmental damage.

But John Pike, director of the military studies group GlobalSecurity.Org, said: "You can call it something other than napalm but it is still napalm. It has been reformulated in the sense that they now use a different petroleum distillate, but that is it. The US is the only country that has used napalm for a long time. I am not aware of any other country that uses it." Marines returning from Iraq chose to call the firebombs "napalm".

Mr Musil said the Pentagon's effort to draw a distinction between the weapons was outrageous. He said: "It's Orwellian. They do not want the public to know. It's a lie."

In an interview with the San Diego Union-Tribune, Marine Corps Maj-Gen Jim Amos confirmed that napalm was used on several occasions in the war.

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