A street vendor sells juices in Aleppo where a teenager was executed for blaspheming (Reuters)

A Syrian teenager has been executed by Islamist rebel fighters for blaspheming in the northern city of Aleppo, an activist group reports.

Muhammad Qatta, 15, was reportedly shot dead in front of a crowd of onlookers, including his parents, for naming the Prophet Mohammed during an argument, the UK-based Syrian Observatory for Human Rights (SOHR) said.

Qatta, a street vendor selling coffee in the Shaar neighbourhood, was abducted and tortured by a rebel group operating in the area, after he was heard saying: "Even if Mohammed comes back to life I won't give it as debt."

Qatta was reportedly replying to another local boy who asked him for a free coffee. The rebel fighters interpreted the sentence as blasphemy and took him away.

Hours later, the 15-year-old was reportedly brought back to the neighbourhood with his shirt covering his head. His body sported marks of torture and beating, SOHR said.

"The rebels then publicly announced that apostasy and cursing the prophet is a terrible vice and that anyone who does so will face similar fate," according to the Observatory.

"They then shot the boy twice with an automatic rifle in front of the gathered crowd, a shot in the neck and a shot in the head, and then left in their car."

A picture of Qatta's face savaged by the bullet wound was posted on SOHR's Facebook page.

The group said his executioners didn't speak the local Syrian dialect but had a classic Arabic accent.

Extremist groups such as Jabhat al-Nusra, which declared its affiliation to al-Qaida and has been designated a terrorist organisation by the US, have been fighting to overthrow the regime of Bashar al-Assad alongside secular rebel forces in the two-year conflict.

Western powers' reluctance to directly arm the rebellion has been partially motivated by the presence of jihadists among the various insurgent actions.

Gruesome acts of cruelty have been witnessed with increasing regularity in the war-torn country, and those perpetrated by rebel groups represent a powerful propaganda tool for the regime, which has maintained it is fighting a terrorist insurgency since the very start of the conflict.

"This kind of criminality is exactly what makes people in Syria fear the fall of the regime," SOHR director Rami Abdel Rahman told Aljazeera.

Last month an international outcry was triggered by an online video depicting a rebel commander cutting the heart out of a soldier's body and biting it.

Meanwhile it has been reported that government troops backed by Hezbollah militants are preparing a major offensive in the north of the country, aimed at retaking the city of Aleppo after the strategic success in the city of Qusayr.