The amateur recording contains graphic images and indicates that the rebels use the same tactics employed by the Syrian army, which have drawn harsh condemnation from human rights groups and governments worldwide.
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In the video, the men are led into a yard full of people chanting "Allahu Akbar (God is great)."
At one point a man in the crowd attempts to stop the filming by putting his hand over the camera lens.
But seconds later the air is filled with the deafening rattle of machine gun fire. After the smoke clears, a pile of corpses where the men had knelt is revealed.
The execution apparently took place in a schoolyard in Aleppo as Assad's forces were shelling the city in an attempt to force the rebels to withdraw.
In the video, which could not be independently confirmed, the men were identified as members of the pro-Assad "shabbiha" militia from the Berri family. At least two of them were in their underwear as they were led down a flight of stairs and lined up in front of a wall.
Gunmen firing on them with semi-automatic rifles continued shooting after the men had fallen to the ground, their bodies piled one on top of another.
The shabbiha (ghosts), have been at the forefront of the crackdown on the 17-month revolt against the ruling Assad family, who are members of the Alawite sect, an offshoot of Shiite Islam.
With secret police support the militia, originally all Alawites, expanded after the uprising erupted in March last year to encompass Sunni recruits drawn mainly from the northern city of Aleppo and the eastern province of Deir al-Zor.
Another video showed rebels gloating triumphantly on Tuesday after taking over a police station in the town of Nayrab southeast of Aleppo.
"Come and see the carcasses that die for the sake of Assad," one rebel says before the camera goes into the police station and shows at least 15 bodies in the garden and inside the building, which is riddled with bullet holes and partly burnt.
One rebel points his rifle at the body of the commander of the station, identified as Lieutenant Ahmad al-Khatib, and fires a bullet that blows the dead Khatib's head off.
"I spit on you, and on the tyrant Bashar al-Assad," the unidentified fighter said.
Reuters contributed to the report