REUTERS

"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 Islamic State 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," the article said.

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.