An al-Qaida offshoot group operating in Syria has apologised for mistakenly beheading a fellow jihadist rebel fighter.

The Islamic State of Syria and Levante (Isis) said its fighters in Aleppo decapitated a wounded rebel Sunni Islamist rebel named Muhammad Fares because they believed he was a supporter of the regime of Bashar al-Assad.

"We call on God to accept Mohammed Fares into his Kingdom and to forgive his brothers that sought to rid us of the enemies of God and our enemies," Isis spokesman Omar Al-Qahatani said, after the mistake was revealed.

Fares was reportedly wounded in fights against government troops and taken to a makeshift hospital in a rebel controlled area of Aleppo.

Isis said he was heard shouting the names of two revered figures in Shiite Islam and asked to be put to death thinking he had been captured by the enemy.

Isis rebels, who are infamous for their brutality, were swift in answering the request.

Disturbing footage posted online by the Syrian Observatory of Human Rights shows two Isis fighters addressing a crowd in Aleppo after the beheading.

One is holding the severed head in one hand and a bloodied knife in the other. The rebels say Fares was an Iraqi Shiite who had volunteered to fight with the regime.

"If they enter they will not distinguish between supporters and opponents. I swear they will rape the men before the woman," the Isis fighter says.

Several people however recognised Fares as an Islamist rebel fighting with a group allied to Isis and the error was unveiled.

A rebranding of al-Qaida's Iraqi branch, Isis has become one of the major players in Syria's civil war and now controls large spans of the territory.

Made of mainly foreign jihadists, Isis fights to create an Islamic caliphate in the old Mesopotamia region now including both Iraq and Syria.

Isis's agenda and cruel methods have put it at odds with moderate rebel groups and Kurdish factions, sparking infights that have fuelled growing chaos and lawlessness in Syria.

In Iraq Isis is believed to be responsible for a spree of ethnic violence that has killed more than 5,000 people since April.