As fighters with the self-proclaimed Islamic State seize parts of Iraq and Syria, one religious minority group has been brutally targeted by ISIS militants: The Yazidis.

Last August, ISIS attacked Yazidi villages in northern Iraq and killed hundreds of Yazidi men. Thousands of other Yazidis fled to escape the rampage, but thousands of women and children were abducted by the extremists and forced into sexual slavery. Many of these women and young girls are still being held by ISIS militants, who have been accused of committing genocide and war crimes against the Yazidi people.

Using undercover footage, a new Frontline documentary presents the first-hand accounts of women who escaped the brutal reign of ISIS.

One of the young women featured in the film, called “Escaping ISIS,” is 18-year-old Nadia. She was an ISIS sex slave who was taken from her family and held for more than 100 days. Though she managed to escape, she says many others did not.

“Two of my friends electrocuted themselves,” she says. “One hanged herself with her shawl. One woman slit her wrist — she went to the bathroom, and we saw the blood leaking from under the door. They said they would feed her to the dogs. That’s the fate of anyone who kills themselves.”

In addition to following escapees like Nadia, the film also chronicles the work of a secret network of informants and contacts that have established a de facto underground railroad to help rescue hundreds of Yazidi women and girls.

“It can take a whole month to rescue one family, or sometimes one person,” says Khalil al-Dakhi, a Yazidi who has helped build this secret network. “It’s very difficult. They are being raped and mistreated on a daily basis. They are calling, crying, are asking us to rescue them, telling us they can’t cope any longer. It’s very painful. There’s no one else they can ask for help because their family is either taken or killed.”

Within ISIS territory, Khalil's network of opposition tries to help people escape by using mobile phones. The phones are smuggled in or are carefully stolen from ISIS militants. Those that are held captive within ISIS territory use the network to send messages to their loved ones and to seek help.

“These mobile phones are a lifeline for people who are trapped inside ISIS [compounds],” says Edward Watts, producer and director of “Escaping ISIS.” He spent two months in Iraq and Turkey filming the documentary, finding undercover footage and talking with individuals like Nadia and Khalil.

“[Khalil’s] an extraordinary individual. He’s the head of a whole team of guys, probably about eight of them in total, who are doing this work,” says Watts. “Through his own ingenuity, he really realized that the information these women were bringing back actually gave him an extraordinary picture inside ISIS — to really get a map of their territory and an understanding of how they were deployed.”

Before ISIS overran his town in northern Iraq, Khalil al-Dakhi was a lawyer, handling legal affairs like marriages, divorces, and other civil issues. As ISIS moved in, Watts says Dakhi decided to use his law skills to collect the testimony of Yazidis who have been targeted by ISIS. From there, the underground rescue network began to take shape.

“As some of the women and girls started to come back [from captivity], he began to gather their testimony — a record of what they experienced,” says Watts. “Initially, I think, he was hoping they would provide evidence for a war crimes trial, which he still hopes to see.”

The testimony painted an in-depth picture of where Yazidi women and girls have been held — sometimes down to the specific building they were kept at.

“That information was obviously incredibly helpful for anyone that was trying to get them out,” says Watts. “That is how this whole process began. [Dakhi] describes it himself as a do-it-yourself operation. They’ve really just done it off their own back, using their own wits to work out how you might get these women out. He’s dedicated his life to doing that, and he’s really a hero for our times I think.”

There are an estimated four million Muslim women living under ISIS control. Though ISIS has imprisoned Christians and Shias in the region, Yazidis face particularly harsh treatment — the terrorist organization views this religious minority group as almost “subhuman,” says Watts.

“It’s that particular categorization that allows them to treat the women as sex slaves, as objects, as chattels basically,” he says.

“The sheer cruelty that these guys are capable of — I’ve made 20 documentaries about lots of horrendous things happening around the world and I’ve never come across anything like this,” he adds.

“Escaping ISIS” airs on PBS Tuesday night at 10 pm.

This story first aired as an interview on PRI's The Takeaway, a public radio program that invites you to be part of the American conversation.