BEIRUT — Al-Qaida-linked jihadists in Syria have admitted beheading a fellow rebel by mistake after believing him to be an Iraqi Shiite fighting alongside regime forces, a watchdog said on Friday.

A video posted on the Internet on Wednesday 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 in northern Syria.

They said he was an Iraqi Shiite who had been fighting among the ranks of President Bashar Assad's forces.

"Some minutes after the video was posted, the man was identified as Mohammed Marroush, a fighter with rebel group Ahrar al-Sham," Syrian Observatory for Human Rights chief Rami Abdel Rahman said.

The Islamist Ahrar al-Sham is an ISIL ally.

"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.

Marroush had been wounded in fighting at a regime military base east of Aleppo, Syria's second city and former commercial hub.

In the battle, rebel and jihadist groups squared up against Syrian soldiers backed by members of Lebanon's Shiite Hezbollah movement and Iraqi Shiites of the Abu Fadl al-Abbas group.

Marroush was taken to hospital outside Aleppo for treatment, and in his drugged state was heard to repeat the names Ali and Hussein, two venerated Shiite imams.

"This was the last thing he had heard from the Shiite fighters before being wounded," an Observatory statement said.

"The two ISIL men deduced he was a Shiite fighter and cut his head off," it added, calling the decapitation "a war crime."