WASHINGTON - Some Syrian rebel fighters are committing human rights abuses as they battle the forces of Syrian President Bashar 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.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-Qaida."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.