A YAZIDI sex slave has described how ISIS terrorists would tell the captives they had to rape them to convert them to Islam.

The unnamed victim also described how the sick thugs would grope the breasts of girls they had captured to see if they were old enough to be raped.

4 ISIS fighters use rape as a form of terrorism against its victims. File picture Credit: AP

Her harrowing testimony was featured in the report Trafficking Terror, released by the Henry Jackson Society think-tank.

She said that Islamic State members would touch the chests of captured girls to see whether they had grown breasts.

If they had breasts, they could be raped, if not, they would wait three months to check again.

She added: "The men used to rape up to seven girls in one room, so everybody could see what was going on; the screaming was up to the skies.

4 ISIS fighters use rape as a weapon. File photo showing captured girls in ISIS territory Credit: Alalam.ir

"They were all screaming but they could not help each other. They had to watch and go through what was happening."

She described how one victim was kept in a room without clothes, gang-raped and was sold over and over to different groups of men.

Once she tried to escape and was raped by six men in one night as a punishment.

The Yazidi woman said female ISIS terrorists would tell captives: "You have to be raped to become a Muslim."

The report also revealed that ISIS is attracting rapists and paedophiles to fight for them by rewarding them with sex slaves.

Men with a history of sexual violence and domestic abuse are believed to have joined the jihadi group because of the organisation’s use of rape and slavery as a form of terrorism.

4 ISIS sells sex slaves to fund its sick cause. Pictured is Khalida, 25, who escaped ISIS Credit: Andy Bush - The Sun

The promotion of sexual violence by the extremist group was a vital means of “attracting, retaining, mobilising and rewarding fighters” as well as punishing disbelievers.

The sexual exploitation of women and children alongside trafficking helped fund the caliphate and was used to lure men from deeply conservative Muslim societies, where casual sex is taboo and dating prohibited, the report claims.

In addition, forced pregnancies - along with forced conversions - were officially endorsed to help secure the next generation of jihadis, a tactic also used by Nigeria’s militant Islamist group Boko Haram.

Analysis of ISIS members from Europe and the US found that some had a history of domestic and sexual violence, suggesting a “relationship between committing terrorist attacks and having a history of physical and/or sexual violence”.

4 Ekhlas told how she was just 14 when she was kidnapped by ISIS fighters and taken as a sex slave Credit: BBC

It adds that propaganda promotes sexual slavery as an incentive for new recruits and foreign fighters, with the promise of wives and sex slaves acting as a sickening "pull factor".

In April a schoolgirl bravely recalled the horror of being kidnapped by ISIS fanatics and raped daily for six months.

Ekhlas was just 14 when Islamic State fighters swept through her village in northern Iraq on August 3, 2014 as they targeted the local Yazidi community, an ethnic Kurdish group and one of the country's oldest minorities.

ISIS falsely accused the Yazidis of being devil worshippers and thousands of people were expelled from their homes.

Many men were shot dead while women and children were taken as hostages and held as sex slaves.

“My life was beautiful but two hours changed my life," Ekhlas, now 16, exclusively told the BBC's Victoria Derbyshire show.

“They came with their black flag. They killed our men and raped our girls.

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