Rebels fighting the Syrian regime have shot dead a 15-year-old boy in front of his parents and siblings after accusing him of blasphemy, an activist group said.

Al Jazeera was told that the boy, a street vendor selling coffee, was from the Shaar neighbourhood of the northern city of Aleppo. He has been named locally as Mohammad Kattaa.

Reports indicated that he was arguing with another boy on Saturday and used the name of Prophet Muhammad in a common phrase used by Syrians at which point he was picked up by fighters, beaten, and then shot.

"An unidentified Islamist rebel group shot dead a 15-year-old child who worked as a coffee seller in 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 foreigners.

"They spoke classical Arabic, not Syrian dialect," he said.

"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.

'Criminal act'

The Observatory condemned the execution as "criminal" and said it was 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.

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

It is thought Kattaa's customer was trying to get a free coffee and the boy responded "Even if Muhammad comes down, I will not give it as debt."

This was misinterpreted by the foreign fighters who took it for blasphemy.

Meanwhile, government troops on Sunday were preparing to launch an offensive in the north of the country.

Syrian daily al-Watan said the army has "started to deploy at a large scale in Aleppo province, in preparation for a battle that will be fought in the city and its outskirts".

Rebels last July launched a massive assault on Aleppo, once Syria's commercial hub. The city has since suffered daily regime bombardment and clashes.