The gash on Abu Ahmed's face has almost healed but the troubling memory of Syrian rebels thrashing him with rifle butts evokes fear of another looming war: an intra-rebel conflict.
Rebel groups battling a common enemy -- the regime of President Bashar al-Assad -- are frequently confronting each other, with ideological rifts emerging in the highly fragmented opposition as Syria's civil war grinds on in its third year.
Ahmed, the commander of a Liwa al-Tawhid Brigade, came under attack last month when angry Free Syrian Army rebels seeking to free a comrade detained by another brigade raided a primary school run by his wife.
Ahmed, mistaken for the head of the other brigade, was knocked unconscious in front of students and staff, and dragged off to their base before the rebels realised they had beaten up the wrong man.
"They apologised and went on to hunt for the FSA man they were after," said Ahmed, a burly-framed commander flanked by Kalashnikov-wielding men from his brigade who now guard the school round the clock.
"The kids were so terrified some of them did not come to school for days. Possessing lethal weapons does not give you the right to behave like warlords," he said at the school, surrounded by shell-ravaged buildings.
Growing incidents like these of intra-rebel skirmishes -- some of them fatal -- highlight an intense contest for power in rebel-controlled areas, with many like Ahmed bracing for what they fear will be another war in post-Assad Syria.
Last month, Abdullah, another Liwa al-Tawhid rebel, was killed while driving to the brigade headquarters. He was shot in the neck. His brother Diaa accuses FSA's Sawt al-Haq Brigade of having orchestrated the killing over a petty personal dispute.
A sharia Islamic court in Aleppo is currently investigating the case.
"This is not a good time for rivalry. This is a time to unite against Assad's regime," Diaa said.
Even more ominous is the growing friction between Islamist fighters and the ostensibly secular-leaning FSA brigades.
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What started as an uprising against four decades of Assad family rule has mutated into a full-scale civil war with no end in sight.
Sunni fighters from neighbouring countries have poured into the country, including veterans of the revolution in Libya and jihadists linked to Al-Qaeda in Iraq, out to create an Islamic state.
That has created unease with indigenous FSA brigades which espouse a more moderate interpretation of Islam.
"As the situation in Syria continues to deteriorate, clear fault lines marking the battle for post-Assad Syria are emerging. In particular, rivalries among different rebel groups are taking shape along ideological lines," said Mona Yacoubian, a senior adviser for the Middle East at the Washington-based Stimson Center.
"Groups with a more moderate, inclusive vision of post-Assad Syria are pitted against more radical extremists with an ultra-conservative interpretation of Islam. Already these groups have engaged in limited skirmishes with each other."
Clashes erupted last month between the moderate-leaning Faruq Brigade and the Al-Qaeda-affiliated Al-Nusra Front in Tal Abiyad, on the border with Turkey, in a sign of festering tensions over ideology, with up to four people reportedly killed.
It was one of the bloodiest skirmishes highlighting the ideological divide.
"The Al-Nusra group and its allies have an agenda that other opposition groups don't share -- creating an Islamic emirate and fighting the global jihad, which makes conflict all but inevitable," said Bruce Riedel, a senior fellow at the Brookings Institution.
"A conflict between rival opposition groups is all but certain in Syria once Assad falls and perhaps even sooner."
Some FSA groups face an internal dilemma over Al-Nusra, unable to ignore their battlefield prowess as the group emerges as one of the strongest in the fight against Assad's forces.
But they are equally uneasy over Al-Nusra's capture of key infrastructure in the country's north and east, including oil and gas plants, a hydroelectric dam and grain silos.
"They (Al-Nusra fighters and allies) have left their homes, their countries to come fight our war," said Abu Basir, an FSA brigade commander in Latakia on the Mediterranean coast, who regularly collaborates with Al-Nusra on the battlefront against Assad.
"But this is our country and we don't want outsiders to come and rule over it. They must realise that they have to leave once the war ends," he said, warning that tensions could spiral otherwise.