A graphic mobile phone video obtained by the BBC appears to show militants from Islamic State (IS) torturing a 14-year-old Syrian boy.
The footage, filmed by a defector from the jihadist group, shows the boy being beaten while he hangs by his wrists.
The UN has accused IS and other armed groups in Syria and Iraq of torturing and killing children.
Children have also been recruited, trained and used on the battlefield.
Another teenager told the BBC how he fought and killed for the al-Qaeda-linked al-Nusra Front at the age of just 15, and that when he switched to IS he found 13-year-olds being indoctrinated.