An Isis terrorist group forced a sex slave to undergo surgery to "restore her virginity" every time she was married to 20 fighters, a United Nations official has said.

Zainab Bangura, special envoy on sexual violence in conflict, has interviewed dozens of sexual abuse victims in the region.

She said sexual violence is being committed strategically, in a widespread and systematic manner, and with a high-degree of sophistication by most parties to the conflict in Syria and Iraq.

The group of Isis (also known as Isil or Islamic State) fighters would strip the victims naked and categorise them before trading them in “slave bazaars” and shipping them to other provinces.

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.

Western fighters join Kurds in battle against Isis Show all 9 1 /9 Western fighters join Kurds in battle against Isis Western fighters join Kurds in battle against Isis Foreign Fighters A 67-year old foreign fighter from Canada, nick-named by Kurdish fighters as Hewal Zinar, poses for a photo on April 19, 2015 Uygar Onder Simsek / AFP Western fighters join Kurds in battle against Isis Foreign Fighters A 23-year old fighter and ex-US marine from the US, nick-named by Kurdish fighters as Hewal Agir, guards a look out point during clashes with Islamic State (IS) group fighters on April 17, 2015, in the outskirts of the north-western Syrian town of Tal Tamr AFP Western fighters join Kurds in battle against Isis Foreign Fighters A 26-year old from the US, nick-named by Kurdish fighters as Hewal Dilsad, shows a picture of his wife and son as he poses for a photo on April 14, 2015, in the outskirts of the north-western Syrian town of Tal Tamr Western fighters join Kurds in battle against Isis Foreign Fighters Ex-US marines aged 23 and 24 from the US, nick-named by Kurdish fighters as Hewal Agir (R) and Hewal Agit, guard a position during clashes with Islamic State (IS) group fighters on April 17, 2015, in the outskirts of the north-western Syrian town of Tal Tamr AFP Western fighters join Kurds in battle against Isis Foreign Fighters A group of international foreign fighters pose for a photo on April 16, 2015, in the outskirts of the north-western Syrian town of Tal Tamr AFP/ Getty Images Western fighters join Kurds in battle against Isis Foreign Fighters A foreign fighter poses for a photo with a machine gun on April 19, 2015, in the outskirts of Tal Tamr AFP Western fighters join Kurds in battle against Isis Foreign Fighters A 28-year old foreign fighter from the UK, nick-named by Kurdish fighters as Hewal Baran, poses for a photo on April 19, 2015, in the outskirts of the north-western Syrian town of Tal Tamr, north of Hasakeh AFP/ Getty Images Western fighters join Kurds in battle against Isis Foreign Fighters A 21-year old from the UK and 23 year-old from the US nick-named by Kurdish fighters as Hewal Sores (L) and Hewal Agir guard a post on April 19, 2015 AFP/ Getty Images Western fighters join Kurds in battle against Isis Foreign Fighters A 67-year old man from Canada and a 40-year old from the UK, nick-named by Kurdish fighters as Hewal Zinar (L) and Hewal Cudi train on April 19, 2015, in the outskirts of the Tal Tamr AFP

The UN report also stated that in February militants fighting in Syria were seeking medical attention to improve their sexual prowess, subjecting their wives to 'brutal, abnormal' sex acts, according to local doctors.

Girls as young as five years old were kidnapped from homes and were raped and abused, according to survivors. Some were returned home when pregnancy occurred, and faced ostracism from their communities.

Women have also been pressured to "restore their virginity" after rape, in a procedure under local anaesthetic to "repair" the hymen using dissolvable stitches.

It aims to reconstruct the membrane so it can be broken again on the woman’s wedding night in cultures where blood is required as proof of virginity.