Human Rights Watch study details torture and executions by Syrian rebels

The admission adds weight to claims made by one opposition activist in the countryside near Homs last month that four men seen by The Daily Telegraph in a mass grave who appeared to have been shot with their hands tied behind their backs had been killed by the Farouq Brigade.

The four, whose bodies had not been claimed and were in an advanced state of decomposition, were snipers with the Shabiha, or Assad regime informal militia, who had been captured and subsequently killed, the activist said.

HRW took care in the report to distinguish between atrocities carried out by disparate volunteer militias such as the Farouq Brigade and the Free Syrian Army, a force largely consisting of army deserters who are supposed to be under the central command of a former regime colonel, Riad al-Assad, now based in Turkey. Most such militias claim to be part of the FSA but are only tenuously under its control.

Another FSA-linked militia, the Kafr Takharim battalion, is thought to have been responsible for the execution of a man shown in another video hanged from a tree, in front of several armed fighters, and also said to be a member of the Shabiha.

However, a man named as "Samih" also claims in the report that the FSA itself is kidnapping people to exchange for prisoners or for ransom.

The report takes the form of a letter addressed to the Syrian National Council, seen as the main exiled opposition political leadership. "Leaders of Syrian opposition groups should condemn and forbid their members from carrying out abuses," the group said. It said there were indications that some attacks were motivated by sectarianism – much of the opposition is Sunni, while Alawis and to a lesser extent Shia are accused of supporting the regime.

There was no initial response from the SNC. As the Assad regime has begun to retake rebel-held areas using a campaign of intense bombardment to which the FSA has little response, and with the international community so far refusing to come to its aid militarily, it is in a quandary as to how to proceed.

The FSA fled rebel-held areas of the eastern city of Deir al-Zour on Tuesday, while Homs and Hama have come under more bombardment.

The overwhelming and indiscriminate force used by the regime has won the opposition worldwide sympathy. But as it prepares for a longer guerrilla war, lasting a year or more, the opposition risks playing into the hands of the regime which has sought to portray its activists from the beginning as "terrorists".