NOTE: This video contains graphic testimony and scenes of torture.
The disturbing footage shows a blindfolded 14-year-old Ahmed beaten and electrocuted by IS militants, as he is hung by his wrists.
Two masked militants interrogate and torture Ahmed as hangs from the ceiling. One man can be seen holding a knife and pistol, while the other carries an AK-47.
The video was obtained by BBC journalist Quentin Sommerville.
"When they electrocuted me, I used to scream calling for my mother," Ahmed told the BBC, after fleeing to neighbouring Turkey. "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."
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"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".
Ahmed, who sold bread for a living in the Syrian city of Raqqa, told the BBC he was tricked into placing a bag carrying a bomb near an IS meeting place.
Though he was captured by the group and sentenced to death for his crime, the executioner released the boy out of pity.
"It's rare that I'm able to sleep," Ahmed said. "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."
BBC journalist Quentin Sommerville said he obtained the footage from an IS militant, who has since defected from the group. The man said they filmed Ahmed's torture for "propaganda purposes".
"I am regretting every moment," the man told the BBC. "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."
