The campaign group, Human Rights Watch, has accused rebels in Libya of looting, arson and the abuse of civilians.
Observers from the New York-based group say they have witnessed some incidents themselves, and have interviewed witnesses to others in territory recently seized by rebels.
A rebel spokesman talking to reporters in Brussels has denied the allegations.
Accusations of abuse by both sides have circulated since the rebellion against Col Muammar Gaddafi began in February.
The latest allegations focus on four towns seized by rebels in the west of the country in the last month: al-Awaniya, Rayayinah, Zawiyat al-Bagul and al-Qawalish.
"The rebel conduct was disturbing," said Fred Abrahams, a special adviser to Human Rights Watch (HRW).
"We documented fairly widespread looting of homes and shops, the burning of some homes of suspected Gaddafi supporters and - most disturbingly - the vandalisation of three medical clinics [and] local small hospitals, including the theft of some of the medical equipment."
He said the Libyan government had committed more serious crimes, but that did not excuse the behaviour of the rebels.
"Our aim is to hold all combatants, all militaries - whether they're organised and states and governments or rebels groups - to the same standards, and it's very much also a warning shot across the bow, because of these other areas they are approaching. We're deeply worried about how they might behave and treat civilians in those areas."
A senior rebel leader has rejected the Human Rights Watch claims.
"This is not the case in the liberated areas," rebel spokesman Mahmoud Jibril told reporters in Brussels.
In the latest development from this sector of the frontline, rebel fighters have said they have retaken al-Qawalish, a village about 100km (60 miles) south of Tripoli.
Earlier, forces loyal to Col Gaddafi seized control after the rebels took to their vehicles and fled without a fight, reversing weeks of steady advances, BBC World Affairs editor John Simpson reports.
Our correspondent says this shows how feeble the rebel forces can be.
Even though they are only an hour's drive from Tripoli, Wednesday's fighting will presumably encourage Col Gaddafi to keep up his resistance for a while longer, he adds.
The retreat came after rebel spies reported a build-up of military vehicles in Garyan, the last major pro-Gaddafi military base before the capital.
But by the evening, the rebels had staged a successful counterattack, spokesman Abdurahman Alzintani told Reuters news agency.