Offering kidnapped women and girls to jihadist fighters as sex slaves or wives helps militant groups lure and retain members, while sex trafficking can bankroll their extremist operations, a UK-based think-tank said on Monday.Groups such as Boko Haram in Nigeria and Islamic State in Syria and Iraq may resort more to such practices as they lose ground militarily, the Henry Jackson Society said in its report."Propaganda on sexual slavery serves as an incentive for new recruits and foreign fighters, with the promise of wives and sex slaves acting as a 'pull factor'," researcher Nikita Malik said in the report."Religious elements are infused into sexual violence practices to skirt around the moral wrongdoing of rape," she said.Since it began its insurgency in 2009, Islamist militants Boko Haram have abducted thousands of girls and women in northeast Nigeria – most notably the more than 200 Chibok girls snatched from their school in April 2014 – with many used as cooks, sex slaves, and even suicide bombers.Boko Haram members would purposely impregnate women and girls to produce the "next generation of fighters", the report said.Similarly, thousands of women and girls were abducted, tortured and sexually abused by Islamic State fighters after the militants rounded up Yazidis in the village of Kocho, near Sinjar in 2014. U.N. investigators estimate more than 5,000 Yazidis have been rounded up and slaughtered and some 7,000 women and girls forced into sex slavery.But a final assault on Islamic State's last line of defense in its former Syrian capital Raqqa began on Sunday.The loss of Raqqa, following its defeat in Iraq's Mosul and its retreat from swathes of territory in both countries, would mark a milestone in the battle to destroy the jihadist group.The report said as groups like Islamic State, also referred to as 'Daesh', and Boko Haram struggle to financially sustain their operations, sex trafficking and kidnapping for ransom could increase."Historical revenue streams, including taxation and oil sales, to groups such as Islamic State and Boko Haram are decreasing," Malik said in a statement."These are being replaced with hostage-taking and ransom efforts, meaning modern day slavery may increase as Daesh struggles to sustain its financial reserves," she said.The report said Islamic State generated up to $30 million in 2016 through kidnapping and abductions.