The woman is young, perhaps 18, with olive skin and dark bangs that droop onto her face. In the Facebook photo, she attempts to smile but doesn’t look at her photographer. The caption mentions a single biographical fact: She is for sale.

“To all the bros thinking about buying a slave, this one is $8,000,” begins the May 20 Facebook posting, which was attributed to an Islamic State fighter who calls himself Abu Assad Almani. The same man posted a second image a few hours later, this one a pale young face with weepy red eyes.

“Another sabiyah [slave], also about $8,000,” the posting reads. “Yay, or nay?” The photos were taken down within hours by Facebook, and it is unclear whether the account’s owner was doing the selling himself or commenting about women being sold by other fighters. But the unusual posting underscores what experts say is an increasingly perilous existence for the hundreds of women who are thought to be held as sex slaves by the Islamic State.

As the terrorist group comes under heightened pressure in Iraq and Syria, these female captives appear to be suffering, too — sold and traded by cash-strapped fighters, subjected to shortages of food and medicine, and put at risk daily by military strikes, according to terrorism experts and human rights groups. Social-media sites used by ­Islamic State fighters in recent months have included numerous accounts of the buying and selling of sex slaves, as well the promulgation of formal rules for dealing with them. The guidelines cover such topics as whether it’s possible to have sex with prepubescent prisoners — yes, the Islamic State’s legal experts say — and how severely a slave can be beaten.

But until the May 20 incident, there were no known instances of Islamic State fighters posting photographs of female captives being offered for sale. The photos of the two unidentified women appeared only briefly before being deleted by Facebook, but the images were captured by the Middle East Media Research Institute, a Washington nonprofit group that monitors jihadists’ ­social-media accounts.

“We have seen a great deal of brutality, but the content that ISIS has been disseminating over the past two years has surpassed it all for sheer evil,” said Steven Stalinsky, the institute’s executive director, using the common acronym for the Islamic State. “Sales of slave girls on social media is just one more example of this.”…