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.
The film was made for propaganda purposes, he said.
The fate of the two other men filmed being tortured at the same time as Ahmed is not known.
"I am regretting every moment," the man said. "When I joined IS, I wasn't convinced of it but I had to."
"Although I wasn't particularly heavy-handed with people, I hope that the people I hurt will forgive me."
Corruption of a generation
Inside its self-declared caliphate, Islamic State has ended secular education and instead created military-style schools which indoctrinate children and train them to kill.
An IS propaganda video shows children, some barely teenagers, undergoing drills and learning to shoot.
Children are shown in beheading videos, and also taking part in killings.
Khaled - not his real name - is now 17, but just two years ago he fought and killed for al-Qaeda's affiliate in Syria, al-Nusra front.
Later, he was forced to join IS. He has since defected, but for him entering battle was a rite of passage to manhood. He said that children were being specifically targeted to fight.
"Adult fighters of the Islamic State are a minority. My graduation class was full of 15 and 16 year olds," he said. "There were even many as young as 13 or 14. Those are more eager to fight and wage jihad for the sake of God."
Khaled bristles when the young recruits are referred to as children.
"May God forgive you, don't say children," he told me. "Those carrying a rifle, seven magazines and six grenades are men. They are Muslim men. There's a difference between being a male and a man."
"Many people are just male and not men. Those taking up arms and fighting for the sake of God and defending Muslim women's honour are men."
The Islamic State has up-ended life in Syria and Iraq, but its effects are more than just immediate.
This is the corruption of a generation. The children of the IS will grow into young men trained to kill and hate and they will trouble Syria and Iraq for years to come.