Fighters from an as-yet-unidentified faction of the Syrian rebels have executed a 15-year-old boy in the city of Aleppo, accusing him of “blasphemy” for a comment he made during a conversation with another teen.

The teen, working as a coffee vendor, was identified as Mohammad Kattaa, and in the conversation reportedly used a colloquial phrase common in Syrian Arabic, but which takes the name of the Prophet Mohammad in vain.

Though the nationality of the rebel fighters is unclear, they were said to be speaking classical Arabic without the normal Syrian dialect. They took Kattaa to his home, and executed him by shooting him in the mouth, in front of his parents and siblings.

Attacks by foreign Islamists, aiming to enforce an extremely strict form of Sharia, are increasingly common in rebel-held parts of Syria, and have many Syrians fearing for what life will be like if the Islamists end up ruling the entire country.

Rebel violence in many cases appears to be non-specific, with reports that rebel fighters in the Golan Heights attacking a news van belonging to Rossiya TV, a Russian media outlet.