Various armed opposition groups offer different visions for how the future country should be led.
Syrian army warplanes have launched airstrikes in the north of the country, leaving scores of people dead, activists have said.
The Local Co-ordination Committees, an opposition activist network, said on Friday that 70 people had been killed in the offensive in the town of Harem in Idlib province.
Syrian government have also intensified air strikes on Eastern Ghouta in the suburbs of the capital Damascus, an activist told the AFP news agency.
Moaz al-Shami said President Bashar al-Assad’s forces had resorted to heavy aerial bombardments after failed attempts by ground troops to seize back the suburbs.
State media has said the military is “cleansing” what it describes as terrorists from the area.
“The regime has tried to control the Eastern Ghouta several times and each times it had to withdraw because the rebels slipped away then came back to attack its forces,” Shami said. “Now it is trying to annihilate it from the air.”
Activists reported the deaths of seven people in airstrikes in the suburb of Douma, amid clashes in the city.
In Idlib province, the Syrian army was reported to have abandoned its last base near the town of Saraqeb, after a fierce assault by rebels, further isolating the strategically important second city from the capital.
Opposition activists said on Friday government troops left the town and surrounding areas “completely outside the control of regime forces”.
The UN said the rebels appeared to have committed a war crime after seizing the base.
The pullout followed co-ordinated rebel attacks on Thursday against three military posts around Saraqeb, 50km southwest of Aleppo, in which 28 soldiers were killed.
Several were shown in video footage apparently being shot after they had surrendered.
“The allegations are that these were soldiers who were no longer combatants,” UN human rights spokesman Rupert Colville said in Geneva.
“And therefore, at this point it looks very likely that this is a war crime, another one.
“Unfortunately this could be just the latest in a string of documented summary executions by opposition factions as well as by government forces and groups affiliated with them, such as the shabbiha [pro-government militia].”
‘Violations damage revolution’
Video footage of the killings appeared to show rebels berating the captured men, calling them “Assad’s dogs”, before firing round after round into their bodies as they lay on the ground.
Rights groups and the UN say rebels and forces loyal to Assad have both committed war crimes during the 19-month-old conflict.
The opposition Syrian National Council (SNC) on Friday urged that rebels be held accountable for human rights violations.
“We urge the [rebel] Free Syrian Army and the revolutionary movement on the ground to hold to account anyone who violates human rights,” SNC human rights committee head Radif Mustafa told AFP by telephone.
“Though the rebel armed groups’ violations are not as serious or numerous as those committed by the regime, we cannot be silent over such violations because that would only help them to increase.
“The FSA and other armed groups need to respect human rights.”
Mustafa described the situation on the ground in Syria as “very complex”.
“There are individual cases of revenge,” he said. “Such violations are very dangerous. Any violation damages the revolution’s principles of freedom, dignity and respect for human rights,” he said.
The conflict in Syria began with protests against Assad and has spiralled into a civil war which the UN says has left 32,000 people dead and which threatens to drag in regional powers.