Mohammad Qataa was shot in the face and neck a day after being seized, said the pro-opposition monitoring group, which is based in Britain and uses a network of observers across Syria.
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"The Observatory cannot ignore these crimes, which only serve the enemies of the revolution and the enemies of humanity," said the group's leader Rami Abdulrahman.
A photo released by the Observatory showed Qataa's face with his mouth and jaw bloodied and destroyed, as well as a bullet wound in his neck.
Teenager was executed in front of parents
The Observatory, which based its report on witness accounts of the killing, said Qataa, who was a street vendor selling coffee in the working-class Shaar neighborhood, had been arguing with someone when he was overheard saying: "Even if the Prophet Mohammad comes down (from heaven), I will not become a believer."
The gunmen, who belong to the Islamic State of Iraq and Syria, a militant group that started off known as the Nusra Front, took Qatta on Saturday and brought him back alive in the early hours of Sunday to his wooden stand, with whiplash marks visible on his body.
"People gathered around him and a member of the fighting brigade said: 'Generous citizens of Aleppo, disbelieving in God is polytheism and cursing the prophet is a polytheism. Whoever curses even once will be punished like this."
"He then fired two bullets from an automatic rifle in view of the crowd and in front of the boy's mother and father, and got into a car and left," the report said.
Abdulrahman said the boy's mother had pleaded with the killers, whose Arabic suggested they might not be Syrian, not to shoot her son. Qataa's parents said the youth had taken part in pro-democracy demonstrations in Aleppo.
Since last year, large parts of the city have fallen under the control of Islamist brigades, including the al-Qaeda-linked Nusra Front, as well as other rebel units.
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