Courtesy Syrian Observatory for Human Rights

A teenager selling coffee in the northern Syrian city of Aleppo was arrested by Islamist rebel fighters for insulting the Prophet Mohammed, beaten and then executed in front of his family, a watchdog group claims.

The boy, Mohammed Qatta, 14, reportedly refused to give a customer coffee, the Syrian Observatory for Human Rights reported Sunday.

"Even if [Prophet] Mohammed comes back to life, I won't," the boy said, who was known by his nickname "Salmo."

Extremist rebels driving past in a black car overheard the comment, the opposition Aleppo Media Center said. Qatta was taken away by the fighters and later brought back, his head wrapped with his shirt and his body covered with marks from whipping.

The rebels then read out the boy's sentence - not in a Syrian accent, but in classical Arabic. They accused the boy of blasphemy and told the crowd - which included the boy's parents - that anyone who insulted the Prophet would suffer a similar fate.

Qatta was then shot in the mouth and neck. A graphic photo was released late Sunday of the dead boy clearly showing wounds that matched the reports.

The boy's parents confirmed the accounts in an interview posted online on Monday by the Aleppo Media Center. In it, his father stoically recounted the execution while his mother wailed.

"Why did they kill my son," she cried. "We are not for or against anybody in this conflict, may God take revenge on them."

The Islamists and their group have yet to be identified. Large swathes of rebel-held Aleppo are under the control of al Qaeda-linked rebel groups who have set up Sharia courts and welcomed large numbers of foreign fighters into their ranks. The United States and its allies have struggled with the question of whether to arm an extremely fragmented opposition force whose strongest elements have pledged allegiance to al Qaeda.

Aleppo is expected to become of the focal point of Syria's two-year civil war in the coming days as Syrian forces look to take back full control of Syria's most populous city in a new offensive reportedly dubbed "Northern Storm."

ABC News' Nasser Atta contributed to this report