Al-Qaeda-linked rebels have said sorry for decapitating a fellow extremist by accident.

Militant Islamist rebels in Syria linked to al-Qaeda have asked for "understanding and forgiveness" for cutting off and putting on display the wrong man's head, The Telegraph has reported.

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Knife-wielding members of Islamic State of Iraq and al-Sham were shown in a video triumphantly displaying a bearded head before a crowd.

But the head was recognised from the video as belonging to a member of Ahrar al-Sham, a Sunni Islamist rebel group that often fights alongside ISIS.

After inquiries, an ISIS spokesman admitted he was the wrong man.

Sheepishly explaining the error rebels made reference to a story in which Mohammed said Allah would forgive a man who killed a believer in error.

The news comes after Al-Qaeda took to Twitter to show users how to "kindly" cut off a human head... because they don't want to seem violent.

An article in the Lebanese daily al-Safir examined al-Qaeda's increasing use of social media networks and media technology in spreading its ideology.