The findings come as the fighters have enlarged the area under their control in the west and inched closer to a key supply route to Tripoli.
NATO has been bombing Qaddafi's forces and military sites to enforce a UN resolution to protect civilians. Still, the civil war has fallen into a virtual stalemate, with neither side able to make significant progress in recent weeks.
The Human Rights Watch report, based on interviews with local fighters and residents, said that after seizing towns, opposition forces burned down a number of homes believed to belong to Qaddafi supporters and carted out supplies from stores and medical facilities.
The spokesman for the National Transitional Council based in Benghazi denied at first that anti-Qaddafi fighters were involved. "These acts were carried out by individuals who don't represent the NTC or the Feb. 17 revolution," he said. He acknowledged, though, that it "could be a mistake" and said if there was evidence, those involved would be brought to justice.
In one case cited by the report, the rights group witnessed five houses on fire in the village of Qawalish, which was seized by the fighters on July 6, and gunmen loading their truck with supplies looted from a shop.
A few days later, nine more houses had been set alight.
HRW said clinics in three other towns were also looted and vandalized. "We basically took everything," a fighter in Awaniya told the New York-based group.
The reports throw NATO in an awkward position, since the stated goal of their air campaign against government forces is to protect civilians. Rights groups have accused Qaddafi's forces of violations, too, including indiscriminately shelling civilian areas, abusing detainees and laying land mines.
On Wednesday, government forces retook Qawalish. Anti-government fighters who pulled back to the nearby town of Zintan said pro-Qaddafi forces swept through Qawalish from the east and reached as far as the checkpoint on the western edge of the village.
In Brussels, a senior Libyan opposition official said they have held no negotiations so far with Qaddafi's regime. "All this talk about negotiations taking place between the regime and the NTC are totally false claims," Mahmoud Jebril, a senior member of the council, told reporters. "There were no negotiations taking place in the past and there are no negotiations taking place right now. There are ideas flying in the air from one capital to another, but no coherent, comprehensive initiative has so far put on the table."
