The video that emerged on social media showed a visibly frightened boy lying in the back of a truck alongside what appeared to be several rebel fighters.
The rebels claimed the boy was a prisoner from Liwa al-Quds -- a pro-regime militia force made up mostly of Palestinians in Syria -- and that he was captured from an unofficial refugee camp north of war-torn Aleppo.
The footage then shows one of the men taking a knife to the child's head. The boy appears to be quite young, perhaps around 10, though CNN cannot verify his age or identity.
But al-Quds denied the boy had anything to do with the group, saying that he was not a prisoner and lived with his family -- "one among a number of poor families living in areas controlled by the terrorists."
In a statement posted on their Facebook page, the rebel group involved, Nour al-Din Zenky Movement, called the killing an "individual mistake" and said it would investigate "the human rights abuses that were shared on social media sites."
The group is engaged in a bloody war against forces loyal to President Bashar al-Assad.
The statement added that such abuses are "individual errors that represent neither our typical practices nor our general policies." It added that the men involved had been detained and turned over to a "judicial committee" appointed to lead the investigation.
'Regime's killing machine'
The group, however, went on to denounce 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."
It added that the "regime's killing machine has claimed the lives of thousands of civilians and has placed a suffocating siege on the liberated areas of Aleppo City."
The Syrian National Coalition, which represents a number of rebel groups aligned against Assad forces, said it was "deeply shocked" by the images. Though it praised Nour al-Din Zenky for condemning the killing, the coalition said it would follow up on "all legal procedures the movement pledged it will take to punish the perpetrators of this crime that must not be tolerated."
But 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, Director of Amnesty International's Middle East and North Africa Program.
Syria's state-run SANA news agency accused Nour al-Din Zenky of being backed by Turkey, linked to the "Erdogan regime," saying that it "facilitates the entry of terrorist mercenaries to Syria through the Turkish border to carry out these crimes."
Ankara has been one of the fiercest critics of the Assad regime, with thousands of Syrian refugees fleeing across its borders since the conflict began five years ago.
The increasingly dire situation in Aleppo -- which has seen tens of thousands of civilians trapped inside the war-torn city amid heavy fighting -- also threatens to deal a blow to Turkey. In recent days, regime forces have recaptured parts of the city
, threatening to cut Turkey off from the opposition forces it backs.
Already shouldering the burden of more than two million refugees, Turkey fears the fall of Aleppo will send more to flee toward the border.