Activists have accused a supposedly moderate rebel group of beheading a boy on camera. Secretary of State John Kerry said the US is "very concerned" about the report.
The Nour el-Din al-Zinki Movement said on Wednesday it was investigating a video depicting the brutal murder of a boy by apparent members of the US-backed rebel group.
In an image reminiscent of those associated with the so-called "Islamic State" (IS) terrorist organization, a boy is seen being beheaded by a fighter with a knife after being taunted on the back of a truck.
At one point, the fighters accuse the boy of fighting for the Syrian government. "This is a prisoner from the Quds Brigade. They don't have men anymore so they've sent us children today," one of the fighters is heard saying, according to Reuters news agency. "These are your dogs, Bashar, children of the Quds brigade."
The world reacts
The UK-based Syrian Observatory for Human Rights said the men were members of Nour el-Din al-Zinki, which quickly condemned the video and referred to the killing as an "individual mistake."
Little is known about the boy on the video, though activists on social media identified him as Mahmoud Issa, a 12-year-old Palestinian. The Quds Brigade denied that Issa was a member of their group, describing the boy as "ill."
Activists and international organizations reacted in disgust to the video, with Amnesty International among those saying the video shows that opposition groups are also committing atrocities.
Kerry says IS on defensive
US Secretary of State John Kerry called the news of the video "appalling," even though it hadn't yet been confirmed as real. Kerry was attending an international conference on Iraq on Wednesday, where he told an audience that momentum had shifted in the battle against IS and the US and its allies were now gaining significant ground.
However, Kerry also stressed that recent victories meant a growing need for assistance in stabilizing this newly acquired territory. "The new challenge that we face is securing and aiding the recovery of a liberated area," he said.
Kerry said the goal was to raise $2 billion (1.8 billion euros) in aid to help those displaced by the ongoing conflict.