Some Syrian rebel fighters are committing human rights abuses as they battle the forces of President Bashar al-Assad, although these incidents pale in comparison to the government's campaign of violence, a senior human rights investigator said on Friday.

Donatella Rovera, an investigator with the rights group Amnesty International who recently spent several weeks in Syria, said it was clear that some opposition supporters had resorted to brutal tactics as they target members of the security forces.

"They capture people, we've seen evidence of them having beaten them up ... and in some cases they have killed them," Rovera told Reuters.

"Can it get much worse? It certainly can."

The strength of the armed opposition is growing, she said earlier in a speech at the Center for Strategic and International Studies, and as clashes intensify, individual members are lashing out and committing human rights abuses by beating, detaining and killing Assad's soldiers.

Amnesty International is tracking these abuses in part by the YouTube videos members of the Free Syrian Army post online when they interview detainees.

Rovera stressed that the government was largely to blame for the escalating violence in Syria, saying Assad's forces have targeted whole villages in their attempt to suppress the spreading rebellion.

In the latest reported massacre, the opposition said, government forces attacked the village of Tremseh in the rebellious Hama region with helicopters, artillery and tanks on Thursday.

Opposition sources put the death toll at anywhere from 100 to more than 200, although no independent account of the attack has surfaced.

Syrian state television blamed the deaths on "armed terrorist groups." Damascus has repeatedly said that it faces a foreign-backed insurgency waged by disparate groups including forces aligned with al Qaeda.

TARGETING MEDICAL TEAMS

Rovera said Amnesty International's reports out of Syria indicated that pro-Assad fighters had in some instances burned up to half of the homes and most clinics in towns to root out rebels, and are increasingly targeting unarmed civilians, including medical teams treating wounded rebels that have been barred from hospitals.

She said three medical aides were found dead in the city of Aleppo.

"The three were arrested and after a week, their bodies were found with clear marks of torture. Their nails had been pulled off, their teeth missing ... and their bodies had been set on fire," Rovera said.

"It was to give a clear message that it is not a good idea to engage in these types of humanitarian tasks."

The U.N. Security Council is expected to vote next week on extending the mandate of U.N. observers in Syria, whose original mission was to monitor a ceasefire that never took hold.

Rovera said it was important that the observers be allowed to stay on to help collect information on rights violations.

"The time for impunity is over," she said.

Violence in Syria has been escalating since protests erupted in March 2011. The unrest has killed more than 17,000 rebels, civilians and government forces, according to the Britain-based Syrian Observatory for Human Rights.