Reuters:

The fatwa was among a huge trove of documents captured by U.S. Special Operations Forces during a raid targeting a top Islamic State official in Syria in May. Reuters has reviewed some of the documents, which have not been previously published.

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."

The United Nations and human rights groups have accused the Islamic State of the systematic abduction and rape of thousands of women and girls as young as 12, especially members of the Yazidi minority in northern Iraq. Many have been given to fighters as a reward or sold as sex slaves.

Far from trying to conceal the practice, Islamic State has boasted about it and established a department of "war spoils" to manage slavery. Reuters reported on the existence of the department on Monday.

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 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?"

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."