Members of an Al Qaeda-linked Islamist group in the Syrian city of Aleppo have allegedly executed a 15-year-old boy in front of his parents as punishment for what the group regarded as blasphemy.

The Syrian Observatory for Human Rights, a Britain-based group opposed to the Syrian regime, says Mohammad Qataa was shot in the face and neck a day after being seized.

It has released a photo showing Qataa's face with his mouth and jaw bloodied, as well as a bullet wound in his neck.

"The observatory cannot ignore these crimes, which only serve the enemies of the revolution and the enemies of humanity," the group's leader Rami Abdulrahman said.

The observatory based its report on witness accounts of the killing.

It says Qataa was a street vendor selling coffee in the working-class Shaar neighbourhood.

He has reportedly 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 observatory says that comment got back to the Islamists, who belong to the Islamic State of Iraq and Syria, a militant group that started off known as the Nusra Front.

The observatory says the Islamists took Qatta on Saturday and brought him back alive in the early hours of Sunday, 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'," the observatory said.

"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."

Mr Abdulrahman says 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 say 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.

More than 94,000 people have been killed and some 1.6 million Syrians have fled the country since the civil war began in March 2011 after a crackdown on protests against the regime of president Bashar al-Assad.

Reuters