This is consistent with Islamic teaching and goes back to Muhammad, "the perfect model." The Qur'an sanction of sex slavery:
If you fear that you will not act justly towards the orphans, marry such women as seem good to you, two, three, four; but if you fear you will not be equitable, then only one, or what your right hands own; so it is likelier you will not be partial. (Qur'an 4:3)This Muslim group, Boko Haram, has been slaughtering Christians and waging war across Nigeria in the cause of Islam. Borno Governor says 'Boko Haram on verge of seizing control of state'
Boko Haram timeline: From preachers to slave raiders BBC May 15, 2013 (thanks to The Religion of Peace.com)
Nigeria's President Goodluck Jonathan has declared a state of emergency in three north-eastern states as militant Islamists intensify their offensive against the government.
The insurgency, led by Boko Haram, has killed some 2,000 people since 2009. It has spread across the mainly Muslim north and central Nigeria.
With the attacks becoming increasingly sophisticated, there is growing concern that Boko Haram is receiving backing from al-Qaeda-linked militants in other countries.
Here, BBC Africa's Farouk Chothia looks at the changing nature of the conflict.First large-scale incursion: May 2013
Boko Haram has increased its focus on smaller towns in north-eastern Nigeria in recent months after the military drove many of its fighters out of Maiduguri - the capital of Borno state, which was the group's main base.
Boko Haram militants have since infiltrated nearby towns, with little resistance from the army.
In a well-planned attack on 7 May 2013 in Bama, some 70km (44 miles) from Maiduguri, about 200 heavily armed men stormed a military barracks, police station and government buildings.
Fifty-five people were killed and 105 prisoners were freed in the raids. Significantly, the militants launched the attack in armoured vehicles mounted with machine guns.
It suggests that they are becoming better-resourced and they can adapt to the changing terrain. Easy to use motorcycles are the trademark for Boko Haram attacks in cities.
The Bama violence came a few weeks after Boko Haram attacked a military patrol in Baga, a nearby town in which it is said to have also built a presence, forcibly recruiting youth into its ranks.
The military retaliated by launching a raid on the town, accusing residents of harbouring the militants.
Nearly 200 people died in the raid, and thousands of buildings were destroyed, leading to claims by rights groups that the military had used excessive force - an allegation it has repeatedly faced as it tries to quell the insurgency launched in 2009.
The army denied the allegation, and blamed Boko Haram for the deaths of 37 people.
First 'slaves captured': May 2013
Boko Haram leader Abubakar Shekau released a video on 13 May 2013, saying Boko Haram had taken women and children - including teenage girls - hostage in response to the arrest of its members' wives and children.
It is unclear whether the hostages are the relatives of government officials or civilians.
Mr Shekau said the hostages would be treated as "slaves", fuelling concern that Boko Haram is adhering to the ancient Islamic belief that women captured during war are slaves with whom their "masters" can have sex.