Syrian rebel beheading
Some of the men who appear in the gruesome beheading video posted on social media sites Screen grab from video

The appalling beheading of a frightened young boy by US-backed Syrian rebels was a "mistake", leaders of the group are now saying.

A video posted on social media shows the terrified boy in the back of a truck near Aleppo with a group of laughing men, one of whom saws at his neck with a small knife. The men then pose with the boy's head.

The victim was identified by activists on social media as Mahmoud Issa, a 12-year-old Palestinian. The Syrian government denounced the killing in a statement to the United Nations, saying the boy was from a refugee camp near Aleppo.

Leaders of the group — the Nour al-Din al-Zinki Movement— now acknowledge that the gruesome murder was an "individual mistake". The group said on its Facebook page that it would investigate "the human rights abuses that were shared on social media sites".

Such abuses are "individual errors that represent neither our typical practices nor our general policies". It added that the men involved in the video had been detained and turned over to a "judicial committee" that will investigate the beheading, reports CNN.

The rebels initially claimed that the boy was from Liwa al-Quds — a pro-Assad militia comprised largely of Palestinians in Syria. But al-Quds denied the boy had anything to do with the group, and said that he was living with his family when he was snatched by the rebels.

The shocking crime by rebels considered "moderate" by their American backers threatens to destroy the alliance between the US and the group. US Secretary of State John Kerry said the news was "appalling", though noted it hadn't yet been confirmed.

"Obviously we're very concerned, certainly if it's accurate," he added. If proven, it would "give us pause about any assistance or, frankly, any further involvement with this group."

A State Department official would not confirm whether the group currently receives US support. But he did tell Reuters: "We routinely vet the groups we work with and support, as you might expect, and their human rights record figures prominently in that.

"We do not support groups that condone this sort of barbarity, period. We note that al-Zinki has said they will investigate this incident, and we hope they do so thoroughly and transparently."

The acknowledgement of a "mistake" may be a bid by the group to salvage US support.

But it wasn't completely humble in its Facebook post, and denounced the international community over "its silence with regards to the war crimes perpetuated by the Assad regime, even though these crimes have reached the pinnacle of criminality and brutality before the eyes of the world".

The Syrian National Coalition, which represents a number of rebel groups, said it was "deeply shocked" by the images. It praised Nour al-Din al-Zenki for condemning the killing, but said it would follow up to punish the "perpetrators of this crime that must not be tolerated".

Amnesty International said the boy's murder was the latest "abhorrent signal that some such groups are carrying out serious abuses with impunity".

"This horrific video showing the beheading of a boy suggests some members of armed groups have truly plumbed the depths of depravity. It is yet another gruesome example of the summary killing of captives, which amounts to a war crime," said Philip Luther, Amnesty's Middle East and North Africa director.