Shocking new ISIS propoganda shows in detail the group's strict teaching on issues from beard length to how to treat women taken as sex slaves.

The documents obtained in former ISIS-held villages in Northern Iraq show in minute detail the jihadis' rules. The colourful posters and pamphlets with the group's logo were found by Reuters in offices allegedly used by the militants until just a few days ago.

The question of how to treat female captives was answered by 32 questions and answers on one pink and red leaflet with another green insert showing how to pray properly.

The documents reportedly taught that "non-Muslim women can be taken as concubines" and senior ISIS clerics have the authority to distribute female slaves among fighters.

Militants can own two sisters as concubines but only have sex with one, the pamphlets said.

"Pre-pubescent girls can be taken as concubines. You cannot have penetrative sex but you can still enjoy them," they added.

Another leaflet states: "Wearing beards is compulsory, shaving is prohibited'' and defines a beard as "hair that grows on your face and your cheeks."

When Islamic State swept through the north of Iraq in 2014, it announced a self-proclaimed caliphate, which appealed to some fellow Sunnis who felt marginalised by the Shi'ite-led central government.

But that appeal faded as Islamic State enforced its medieval thinking with brute force, beheading anyone deemed an opponent.

Slick, colorful posters, pamphlets and documents highlight Islamic State's intense focus on dictating what it called proper Islamic behavior for the citizens of its caliphate.

Violations of its rules meant punishment such as public whipping or being hauled off to Mosul for execution, according to several villagers who recently escaped from Islamic State areas.

A US-led coalition has forced ISIS to retreat and a largescale campaign by Iraqi troops looks set to push the group of their stronghold in the northern city of Mosul.

Additional reporting by Reuters.