KABUL - The U.S. military denied on Sunday it had used the incendiary chemical white phosphorus in a battle that killed civilians in west Afghanistan last week, after a rights group said it was investigating suspicious burns.

The incident in western Farah province has sparked an outcry over civilian casualties. Afghan President Hamid Karzai has said U.S. strikes killed more than 100 and possibly as many as 130 civilians, and has called for a halt to all U.S. air strikes.

The U.S. military has acknowledged some civilians died, but says the number is smaller and blames Taliban militants for deliberately putting them in harm's way.

Nader Nadery, a member of Afghanistan's independent human rights commission, said doctors who had treated victims from the incident had reported strange burns they believed may have been caused by a chemical like white phosphorus.

White phosphorus, which erupts in flame on contact with air and can stick to flesh causing severe burns, is legal on the battlefield to create light or smoke and is not banned by treaties that forbid using chemicals as weapons.

But its use in populated areas to smoke out enemy fighters has been a persistent source of controversy.

U.S. military spokesman Colonel Greg Julian said U.S. forces used no white phosphorus in last week's incident.

There was no smoke or illum (illumination) used in Farah, he said. I can't say whether the insurgents used it, but we certainly didn't.

Nadery, whose commission is Afghanistan's main rights body, said patients had burns on bodies and face, and the doctors have said it was something not usual from a bomb explosion.

He said the rights commission was alerted to the possibility that white phosphorus munitions may have caused the burns in Farah by a Reuters report last week of a young girl burned by the chemical in another part of the country, the first known account of a casualty from white phosphorus in Afghanistan.

U.S. military doctors who treated that girl had confirmed she was struck by white phosphorus in Kapisa province. The military said it believed none of its shells had struck her house and a Taliban mortar may have been to blame, but rights groups called for the military to reveal more details of its investigation.

Since Reuters disclosed the Kapisa case, the military has released accounts of four incidents it says show that the Taliban use white phosphorus in Afghanistan.

The Farah incident has stoked Afghan anger at a time when thousands of U.S. reinforcements are arriving in the country. Hundreds of students marched at Kabul University on Sunday chanting Death to America and demanding an end to air strikes.

Julian said the anger should be directed at the Taliban, who he said had deliberately kept civilians in houses they were shooting from -- knowing they would be bombed -- and may have also used grenades to kill them.

They wanted to create a civilian casualties crisis, he said. They weren't used as human shields. They were used as human sacrifice.