The camp, which held about 24,000 people, mainly from Syria and Iraq along with more than 6,000 foreign nationals from about 40 countries, was visited in August last year by Dr Lilla Schumicky-Logan, deputy executive director of the Geneva-based Global Community Engagement and Resilience Fund (GCERF).

She warned in a report that the “foreigner annex” was the biggest problem for those running the camp, as the women there were the “most radicalised”, were “angry and highly violent” and a week earlier had burned down three humanitarian offices used to educate their children, dubbed “cubs of the caliphate” by ISIS.

“Gender-based violence, misuse of young boys to impregnate women to uphold the bloodline of the caliphate, and the radicalisation of the ‘cubs’ is ongoing,” she wrote in the report.

The following month Kurdish official Sheikhmous Ahmed told Kurdish media outlet Rudaw Dr Schumicky-Logan’s account of the sexual abuse of teenage boys – who are supposed to be moved away from their families when they reach puberty – was accurate.

“Yes, what she said is true… This is especially the case in the section of the camp which houses foreigners. When children, mostly those aged 15-16, reach puberty age, they hide them and then marry them off to more than a woman (sic). They want to expand their generation,” Mr Ahmed said.

“These women dig tunnels in the camp to hide them. This is done with ISIS approval rather than being an independent act. As per our rules and those of the organisations and the global coalition, those children aged between 12 and 13 should be separated [from their families].”

Mr Ahmed also warned that ISIS-affiliated women in the al-Hol camp “train their children in the camp on weapons, war, killing, and beheading”, and another official confirmed four teenagers were among 15 people arrested in an anti-ISIS raid in early September after tents were burned and security forces attacked.

Al-Hol has now been closed by Syrian officials, with many of the remaining residents transferred to a camp in Akhtarin in northern Aleppo province.