ISIS theologians have issued an extremely detailed ruling on when "owners" of women enslaved by the extremist group can have sex with them, in an apparent bid to curb what they called violations in the treatment of captured females.

The ruling or fatwa has the force of law and appears to go beyond the Islamic State's previous known utterances on the subject, a leading ISIS scholar said. It sheds new light on how the group is trying to reinterpret centuries-old teachings to justify the sexual slavery of women in the swaths of Syria and Iraq it controls.

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Among the religious rulings are bans on a father and son having sex with the same female slave, and the owner of a mother and daughter having sex with both. Joint owners of a female captive are similarly enjoined from intercourse because she is viewed as "part of a joint ownership."

In an April report, Human Rights Watch interviewed 20 female escapees who recounted how Islamic State fighters separated young women and girls from men and boys and older women. They were moved "in an organized and methodical fashion to various places in Iraq and Syria." They were then sold or given as gifts and repeatedly raped or subjected to sexual violence.

The recently uncovered fatwa starts with a question: "Some of the brothers have committed violations in the matter of the treatment of the female slaves. These violations are not permitted by Sharia law because these rules have not been dealt with in ages. Are there any warnings pertaining to this matter?"

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It then lists 15 injunctions, which in some instances go into explicit detail. For example:

"If the owner of a female captive, who has a daughter suitable for intercourse, has sexual relations with the latter, he is not permitted to have intercourse with her mother and she is permanently off limits to him. Should he have intercourse with her mother then he is not permitted to have intercourse with her daughter and she is to be off limits to him."

The Islamic State's sexual exploitation of female captives has been well documented, but a leading ISIS expert at Princeton University, Cole Bunzel, who has reviewed many of the group's writings, said the fatwa went beyond what has previously been published by the militants on how to treat female slaves.

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"It reveals the actual concerns of ISIS slave owners," he said in an email.

Still, he cautioned that not "everything dealt with in the fatwa is indicative of a relevant violation. It doesn't mean father and son were necessarily sharing a girl. They're at least being 'warned' not to. But I bet some of these violations were being committed."

The fatwa also instructs owners of female slaves to "show compassion towards her, be kind to her, not humiliate her, and not assign her work she is unable to perform." An owner should also not sell her to an individual whom he knows will mistreat her.