Al Qaeda-linked jihadists in Syria have reportedly admitted beheading a rebel fighter by mistake.
They apparently believed he was an Iraqi Shiite fighting alongside President Bashar al Assad's forces.
A video posted on the internet on Wednesday allegedly showed two members of the Islamic State of Iraq and the Levant (ISIL) holding up a bearded man's head before a crowd in Aleppo, according to the Syrian Observatory for Human Rights.
The UK-based watchdog's chief Rami Abdel Rahman said: "Some minutes after the video was posted, the man was identified as Mohammed Marroush, a fighter with rebel group Ahrar al Sham.
"ISIL later admitted the rebel had been killed by mistake and said it had arrested one of its men, a Tunisian, for decapitating him. He was referred to their Islamic court."
The second man, also a foreign fighter and from the Gulf, has not been detained.
The Islamist Ahrar al Sham is an ISIL ally.
The fighter had been wounded in fighting at a regime military base east of Aleppo, Syria's second city and former commercial hub.
He was taken to hospital outside Aleppo for treatment, and in his drugged state was heard to repeat the names Ali and Hussein, two revered Shiite imams.
"The two ISIL men deduced he was a Shiite fighter and cut his head off," the watchdog added in a statement, calling the decapitation "a war crime".
It comes as the world's chemical weapons watchdog, the Organisation for the Prohibition of Chemical Weapons (OPCW), met in The Hague on Friday to approve a tight timetable for ridding Syria of its entire deadly poison gas and nerve agent arsenal by mid-2014.
Under a Russian-American brokered deal, Damascus agreed to destroy all its chemical weapons after Washington threatened to use force in response to the killing of hundreds of people in a sarin attack on the outskirts of Damascus on August 21.
The US and Western allies accuse Syria's government of being responsible, while Damascus blames the rebels.
Exactly where the weapons will be taken remains unclear, but Albania, which successfully destroyed its own poison gas arsenal, is being tipped as a candidate, triggering protests there.
Syria's conflict has killed more than 120,000 people in the past two years, according to activists, and displaced millions more.