Islamic State has started selling sex slaves on Facebook in a desperate attempt to raise much needed funds.

The cash-strapped terror group has had its income stream devastated by sustained bombing of its oil refineries and smuggling routes.

Now the group is looking for other, more barbaric ways of paying its fighters and funding its reign of terror.

“To all the bros thinking about buying a slave, this one is $11,000,” said a May 20 Facebook posting, attributed to a German Islamic State fighter who calls himself Abu Assad Almani.

The woman for sale was thought to be 18, with olive skin and dark hair, The Washington Post reports.

Another woman being sold, also for $11,000, had been crying when her photo was taken.

“What makes her worth that price? Does she have an exceptional skill?” one IS sympathiser asked about the woman in the second photo.

"Nope," Alamani replied. "Supply and demand makes her that price."

Facebook reportedly shut down the account within hours, and is playing a cat-and-mouse game with jihadist accounts which are spouting propaganda and organising recruitment and calling for donations.

More than 700 sex slaves have been treated by UN doctors, after being liberated or escaping the group.

Most are Yazidi, a Kurdish minority long persecuted and shunned in Iraq for being "devil worshippers".

Islamic State's territory has shrunk by 40 percent in Iraq and 20 percent in Syria since its 2014 peak.

Iraqi troops advancing towards Fallujah. (Getty)

Thousands of elite Iraqi troops are poised to assault one of the Islamic State group's most emblematic bastions, Fallujah.

The week-old operation has so far focused on retaking villages and rural areas around the city, which lies 65km west of the capital Baghdad.

The jihadists were also under pressure from Kurdish fighters east of their northern Iraqi stronghold Mosul and from US-backed Kurdish-led fighters in Syria.