NEW YORK — A senior United Nations official says Islamic State is circulating a slave price list for captured women and children, and that the group’s ongoing appeal and barbarity pose an unprecedented challenge.

The official, Zainab Bangura, said that on an April trip to Iraq she was given a copy of an Islamic State pamphlet, which included the list, showing that captured children as young as 1 year fetch the highest price. The bidders include the group’s own fighters and wealthy Middle Easterners.

Islamic State has made a particular practice of enslaving communities it has conquered that are not Sunni Muslim – Yazidis and Christians, for example. It portrays such conquests as God’s work, drawing disaffected Muslims from around the world.

The list, showing the group’s view of the value of those it captures, surfaced some eight months ago, though its authenticity came under question. Bangura, the UN special envoy on sexual violence in conflict who was also in Jordan and Turkey, said she has verified that the document came from Islamic State and reflects real transactions.

“The girls get peddled like barrels of petrol,” she said in an interview last week in New York. “One girl can be sold and bought by five or six different men. Sometimes these fighters sell the girls back to their families for thousands of dollars of ransom.”

For Islamic State fighters, the prices in Iraqi dinars for boys and girls ages 1 to 9 are equal to about $165, Bangura said. Prices for adolescent girls are $124, less for women over 20.

The militia’s leaders take those they wish first, then rich outsiders from the region are permitted to bid thousands of dollars, Bangura said. Those remaining are then offered to the group’s fighters for the listed prices.

Bangura, a Muslim and former foreign minister of Sierra Leone, said that Islamic State, which rules some 80,000 square miles across swathes of Iraq and Syria, is unlike other insurgent groups and challenges all known models of fighting them.

Officials and scholars have struggled to understand Islamic State’s success despite breaking what are widely seen as rules for insurgents – to be sure to mingle with local populations, not take on established militaries or try to hold territory.

The group has broken all those rules and draws thousands of foreign fighters despite its well-publicized savagery.

Kerry Crawford, who teaches at James Madison University in Harrisonburg, Virginia, said that publicizing the violations is used to the group’s advantage by building internal ties and external fear.

“If you and your group are doing something that is considered taboo, your doing it together forms a bond,” she said. “Sexual violence does really create fear within a population.”