A Syrian teenager was killed by anti-government rebels in Aleppo early Sunday morning after allegedly misusing the name of Islam's reputed prophet, the Syrian Observatory for Human Rights says.

The human rights group produced grisly photos of 15-year-old Muhammad al-Qatta's corpse and testimony from his family members, who said he worked as a coffee salesman at a street-side shop.

Accounts of what exactly Qatta said differ. According to a post on the Observatory's Facebook page he said, "even if Muhammed comes back to life I won't lend." The same group was quoted by Australia's ABC and other publications as saying Qatta's offense was the phrase, "Even if the prophet Mohammad comes down (from heaven), I will not become a believer."

The Observatory alleged Monday that the killers were associated with a "Sharia committee" of the Islamic State of Iraq and al-Sham, a name al-Qaida uses for its operations in the area. Qatta's father has filed a complaint with the local Sharia committee.

SOHR: Photo of Muhammad Qatta during a protest in Aleppo city. He was killed by foreign jihadists in #Aleppo city. twitter.com/TaziMorocco/st… — X (@TaziMorocco) June 9, 2013

It's unclear if the Aleppo al-Qaida affiliate has responded to news reports. Before allegedly shooting the teen, the gunmen said, "Generous citizens of Aleppo, disbelieving in God is polytheism and cursing the prophet is a polytheism. Whoever curses even once will be punished like this," according to an Observatory statement quoted by ABC.

In a Monday statement, the National Coaltion of Syrian Revolution and Opposition Forces denounced the reported killing.

"If this report is substantiated, the Syrian Coalition declares that this act would constitute a crime against humanity and those responsible must be brought to justice," the Istanbul-based opposition group said. "The Syrian Coalition expects those taking part in the revolution to abide by the ideals and principles of international covenants and treaties."

SOHR: Muhammad Qatta's coffee stand in Shaeer. The child has been executed by foreign jihadists in #Aleppo. #Syria twitter.com/TaziMorocco/st… — X (@TaziMorocco) June 9, 2013

The Syrian Coalition includes mostly moderate anti-Assad fighters and political dissidents. But major rebel groups such as Jabhat al-Nusra, al-Qaida and smaller fundamentalist groups are also very active in the 2-year-old war.

Qatta's killers were speaking Arabic without a Syrian accent, according to witnesses – including family members of the victim – interviewed by the Observatory. Many of Syria's most aggressive rebels are jihadists who have migrated from Iraq.

The activist group that documented Qatta's killing is an equal-opportunity critic. It attempts to compile daily tallies of the people killed in Syria and highlights apparent Syrian rebel and government-committed crimes.