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.
The mobile phone footage shows Ahmed hanging a foot or so off the ground.
He is blindfolded, and two masked men, dressed head to toe in black, pace the ground in front of him. One has a knife and a pistol; the other strides around the room with an AK-47.
They hung Ahmed from the ceiling by his arms and then the beating started.
He was just 14 when IS captured and tortured him.
From the safety of neighbouring Turkey, he relived the nightmare.
“I thought about my parents. I thought about my mum,” he said.
I thought I was going to die and leave my parents, my siblings, my friends, my relatives all behind. I thought I was going to die.
They started lashing me, electrocuting me in order to confess. I told them everything.
In the IS-controlled northern Syrian city of Raqqa, Ahmed had sold bread to make a living.
Two men he knew asked him to put a bag near an IS meeting place. Ahmed had been duped into planting a bomb.
His age was unimportant to his torturers. The abuse lasted two days.
When they electrocuted me, I used to scream calling for my mother,” he said. “But as soon as I did, [one of the torturers] used to up the voltage even more. ‘Don’t bring your mother in it,’ he used to say.
They pretend they’re religious, but they’re infidels. They used to smoke. They pretend to be enforcing the rules of Muslims, but they’re not. They hit and kill people.
From prison, Ahmed was sentenced to death. But his executioner took pity on him, and allowed him to escape.
It’s rare that I’m able to sleep,” he explained. “When I first came to Turkey, I used to have nightmares all the time. I got some treatment. But I couldn’t sleep – I used to dream about it all the time.
Whenever I closed my eyes, I used to have nightmares then stay up all night.
I met the man who filmed Ahmed’s beating. He has since defected from IS and says he is full of remorse.