Dozens of the prisoners bear scars, missing limbs and disfigurements from the battlefield.

Inside a prison full of alleged ISIS fighters

Dozens of accused ISIS militants sit in a crowded cell at a prison run by the U.S.-backed Syrian Democratic Forces (SDF) militia in northeast Syria.

 The SDF guards told CBS News they face regular escape attempts, and they need help managing the burden.

CBS News' Holly Williams was surprised to find prisoners who said they are Americans.

CBS News saw prisoners from their early teens to men whose hair and beards were fully grey. 

Dozens of the prisoners bear scars, missing limbs and disfigurements from the battlefield. ISIS was hammered for months in Syria by SDF forces and U.S. and allied airstrikes. 

The SDF told CBS News that it runs multiple prisons like this one across northern Syria, housing a total of 12,000 accused ISIS militants. 

Nearly all of them want to return to their home countries, even knowing they could face long prison sentences there, but so far their home countries have resisted taking them back.

An SDF guard on duty at the prison in northeast Syria. The militia would not permit CBS News to film the exterior of the facility or reveal its exact location, due to security concerns in the still-volatile region.

Lirim Sylejmani told CBS News he left his home in Chicago to live under ISIS in 2015, and he's unrepentant. 

"The choices that I made, in somebody's eyes, the wrong choices, so I face jail time," he said. 

But he doesn't believe he made the wrong decision, saying he "just wanted to live under Islamic law."

CBS News' Holly Williams met prisoners from across Europe and elsewhere, comparing the mix of nationalities to the United Nations - only every single one of the inmates was accused of joining a terrorist organization. 

She said it was impossible to discern which of the detainees could still pose a threat if released, or if they manage to escape.

The SDF guards told CBS News they face regular escape attempts, and they need help managing the burden. 

They said if the prisoners' home nations won't take them back, they should provide more resources to secure the facilities against mass escape attempts, or establish an international court in Syria to try them.

Dozens of accused ISIS militants sit in a crowded cell at a prison run by the U.S.-backed Syrian Democratic Forces (SDF) militia in northeast Syria.

 The SDF guards told CBS News they face regular escape attempts, and they need help managing the burden.

CBS News saw prisoners from their early teens to men whose hair and beards were fully grey. 

Accused ISIS militants sit in a cell at a prison run by the U.S.-backed SDF militia in northeast Syria.

The SDF guards told CBS News they face regular escape attempts, and they need help managing the burden. 

The militia would not permit CBS News to film the exterior of the facility or reveal its exact location, due to security concerns in the still-volatile region where ISIS remnants remain active.

The SDF guards told CBS News that if the prisoners' home nations won't take them back, they should provide more resources to secure the facilities against mass escape attempts, or establish and international court in Syria to try them.

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