Documents and posters obtained from villages held by Isis have revealed a strict system of rule by the militants, spelling out required beard lengths and guidelines for taking women as sex slaves.

Colourful posters and pamphlets, found in offices that were being used by Isis just days before, reportedly bear the group's logo and go to great lengths to explain their extremist ideology.

Members of the Iraqi forces told Reuters the documents originated from Isis, although this could not be independently verified.

One of the pamphlets reportedly states: ''Wearing beards is compulsory, shaving is prohibited'' before defining that a beard is “hair that grows on your face and your cheeks.”

Another green wallet-size insert lays out guidelines for how to pray properly, showing an image of a young boy undertaking ablutions and reading: “Wash your feet from the direction of your toes down to your heels,” according to Reuters.

A pink and red pamphlet is said to include 32 questions and answers on how to deal with female captives, including a rule stating senior Isis clerics may distribute female captives among its fighters.

The leaflet reportedly reads: "Non-Muslim women can be taken as concubines," before adding that militants can own two sisters as concubines but only have sex with one.

It is also said to state that underage women and girls can be sexually exploited, stating: "Pre-pubescent girls can be taken as concubines. You cannot have penetrative sex but you can still enjoy them.”

Under Isis's rules women are required to stay at home or wear head-to-toe black coverings if they went out. Men meanwhile wear shorts that were deemed Islamic along with beards of appropriate shape and length.

Iraqi Christians celebrate liberation from Isis Show all 10 1 /10 Iraqi Christians celebrate liberation from Isis Iraqi Christians celebrate liberation from Isis Iraqi Christians celebrate liberation from Isis An Iraqi Christian prepares for the first Sunday Mass at the Church of the Immaculate Conception in Qaraqosh Reuters Iraqi Christians celebrate liberation from Isis Iraqi Christians celebrate liberation from Isis Iraqi priests hold the first Sunday mass Reuters Iraqi Christians celebrate liberation from Isis Iraqi Christians celebrate liberation from Isis Iraqi Christian soldiers attend the first Sunday mass at the Grand Immaculate Church since it was recaptured from Islamic State in Qaraqosh Reuters Iraqi Christians celebrate liberation from Isis Iraqi Christians celebrate liberation from Isis An Iraqi Christian soldier holds his weapon during the first Sunday mass Reuters Iraqi Christians celebrate liberation from Isis Iraqi Christians celebrate liberation from Isis An Iraqi Christian soldier lights a candle Reuters Iraqi Christians celebrate liberation from Isis Iraqi Christians celebrate liberation from Isis Iraqi priests hold the first Sunday mass at the Grand Immaculate Church since it was recaptured from Islamic State Reuters Iraqi Christians celebrate liberation from Isis Iraqi Christians celebrate liberation from Isis Iraqi priests hold the first Sunday mass Reuters Iraqi Christians celebrate liberation from Isis Iraqi Christians celebrate liberation from Isis Iraqi Christian soldiers attend the first Sunday mass at the Grand Immaculate Church since it was recaptured from Islamic State in Qaraqosh, near Mosul in Iraq October 30, 2016. REUTERS/Ahmed Jadallah - RTX2R245 Reuters Iraqi Christians celebrate liberation from Isis Iraqi Christians celebrate liberation from Isis An Iraqi Christian soldier attends the first Sunday mass at the Grand Immaculate Church since it was recaptured from Islamic State in Qaraqosh, near Mosul in Iraq October 30, 2016. REUTERS/Ahmed Jadallah - RTX2R256 Reuters Iraqi Christians celebrate liberation from Isis Iraqi Christians celebrate liberation from Isis Iraqi Christian soldiers attend the first Sunday mass at the Grand Immaculate Church since it was recaptured from Islamic State in Qaraqosh, near Mosul in Iraq October 30, 2016. REUTERS/Ahmed Jadallah - RTX2R269 Reuters

Iraqi security forces and Kurdish peshmerga fighters have so far seized several villages and towns during an offensive against the northern city of Mosul, Isis's last stronghold in the country.

Isis is trying to move up to 25,000 people into Mosul as they prepare for what is likely to be an intense battle for control of the city now that Iraqi coalition forces have reached its outskirts.