Islamist rebels in Syria behead 12-year-old boy
More than 280,000 people have been killed and millions displaced since Syria's civil war erupted with the brutal repression of anti-government protests in 2011 (AFP Photo/Omar haj kadour)

Syrian residents and the opposition expressed outrage on Wednesday at the brutal beheading of a child by a rebel group in Aleppo city, branding it a "heinous act".

The horrific murder, captured in footage circulated on social media Tuesday, was carried out by Islamist rebels who accused the boy of fighting alongside government forces.

"How could they slaughter a child like this?" asked 25-year-old Bassel Zein, a barber in the opposition-held Al-Kalasseh neighbourhood of Aleppo.

"He should have been tried in a fair way and maybe swapped with the regime for rebel detainees instead of this heinous act," he said.

Other residents said the beheading tarnished the name of the opposition, and at an Aleppo mosque on Tuesday night, Imam Mohamed Badawi railed against the killing.

"This is the work of criminals, and is forbidden in Islam," he told the congregation gathered for sunset prayers.

The boy was decapitated by members of the Nureddin al-Zenki rebel group on the back of a pick-up truck on a public road in Aleppo's opposition-controlled Al-Mashhad neighbourhood.

In the grisly footage, rebels accused the boy of fighting with a pro-regime Palestinian faction known as the Al-Quds Brigade and said he was captured during fighting north of Aleppo city.

But the Al-Quds Brigade issued a statement denying the boy was a member, and describing him as an ordinary 12-year-old Palestinian refugee.

- 'Individual mistake' -

The Nureddin al-Zenki group condemned the act as a "violation" in a statement on Tuesday, describing it as an "individual mistake that does not represent the general policy of the group".

The organisation said it had detained those involved in the murder and set up a judicial committee to investigate the incident and issue a verdict as soon as possible.

The killing was also condemned by the opposition Syrian National Coalition, which expressed "shock at the horrific scenes" and urged the faction to follow through on its pledge to investigate and punish the perpetrators.

"The National Coalition... does not condone any behaviour that is contrary to the principles of the revolution and the aspirations of the Syrian people who came out to demand freedom, dignity and justice," the coalition added in a statement.

Nureddin al-Zenki is an Islamist rebel group largely based in Aleppo province that at one point received US-made anti-tank missiles, though that support appears to have dried up in 2015.

The faction had already been accused of abuses, with Amnesty International earlier this month saying it was among several rebel factions committing war crimes and other violations of international law.

In Washington, the State Department said it was aware of reports of the "appalling" incident and that involvement with the group could be reviewed.

"We're seeking more information," spokesman Mark Toner said at Tuesday's daily briefing.

"We'll look at, frankly, any affiliation or cooperation with this group we may have going forward, if these allegations are proven true."

- 'Depths of depravity' -

Amnesty International said Wednesday that the beheading was the latest "abhorrent" sign that some opposition groups are carrying out serious abuses with impunity.

"This horrific video... suggests some members of armed groups have truly plumbed the depths of depravity," said Philip Luther, head of Amnesty’s Middle East and North Africa programme

"It is yet another gruesome example of the summary killing of captives, which amounts to a war crime," he said in a statement.

In eastern parts of Aleppo, now under siege after being encircled by government forces, residents accused the rebels involved of besmirching the opposition's name.

"This is a big error and will harm all the Free Syrian Army rebel factions," said Mohamed Mansour, a resident of Al-Mashhad where the execution occurred.

"There is nothing in Islam that allows the slaughter of prisoners. Islam requires captives to be well-treated, fed and cared for."

Rights groups say war crimes have been committed by all sides in Syria's conflict, which began in March 2011 and has killed more than 280,000 people.

The government is accused of widespread torture, as well as indiscriminate fire on civilian populations.

And the Islamic State jihadist group has regularly carried out gruesome executions of its opponents.