The group paraded and traded Syrian and Iraqi girls in 'slave bazaars' before the victims were shipped to other provinces, it was claimed.

Zainab Bangura, special envoy on sexual violence in conflict, travelled to five countries and interviewed dozens of women and young girls who had survived brutal sexual abuse.

She said the girls were routinely stripped naked before being categorized and shipped off.

'Women and girls are at risk and under assault at every point in their lives,' she said.

'ISIL have institutionalized sexual violence and the brutalization of women as a central aspect of their ideology and operations, using it as a tactic of terrorism to advance their key strategic objectives.'

She added that the threat of brutality followed them 'every step of the way… in the midst of active conflict, in areas under control of armed actors, at check-points and border crossings and in detention facilities.'

Ms Bangura's research was completed between 16 to 29 April. She visited Syria, Iraq, Turkey, Lebanon and Jordan.

It is not the first time sexual violence by ISIS fighters has been exposed.

In February it was reported militants fighting in Syria were seeking medical attention to improve their sexual prowess and subjecting their wives to 'brutal, abnormal' sex acts, according to local doctors.

Yazidi women and girls, some as young as five, taken from their homes earlier in the year were also regularly raped and abused by fighters, survivors said.

Some victims even faced being returned home after falling pregnant by their captors and are at risk at being ostracized by their community, which frowns upon pre-marital sex.

A human rights office also published a horrifying report earlier in the year describing killings, torture, rape, sexual slavery and the use of child soldiers by the extremists, suggesting they may be guilty of 'war crimes, crimes against humanity and genocide'.