ISLAMIST rebels fighting the Syrian regime have shot dead a 15-year-old youth in front of his parents and siblings after accusing him of blasphemy, a monitoring group says.

"An unidentified Islamist rebel group shot dead a 15-year-old child who worked as a coffee seller in (the northern city of Aleppo), after they accused him of blasphemy," said Syrian Observatory for Human Rights director Rami Abdel Rahman.

Abdel Rahman said the rebel group likely comprised foreign jihadists. "They spoke classical Arabic, not Syrian dialect," he told AFP.

"They shot the boy twice - once in the mouth, another in his neck - in front of his mother, his father and his siblings," he added.

The Observatory condemned the execution as "criminal and a gift to the regime of Syrian President Bashar al-Assad.

"This kind of criminality is exactly what makes people in Syria fear the fall of the regime," Abdel Rahman said.

The Britain-based monitoring group, which relies on a broad network of activists, doctors and lawyers in Syria for its reports, demanded the killers' arrest.

"We are working on identifying their names," said Abdel Rahman.

Large swathes of Aleppo city have since last year fallen into rebel hands.

Activists have frequently lashed out against rebel groups which have taken advantage of the security vacuum in Aleppo to commit rights abuses.