Now it's clear: The Islamic State (ISIS) is selling captive Christian and Yazidi girls and women — even young children — as sex slaves "in the name of Allah" with the proceeds going to its war chest.

United Nations officials have authenticated a report submitted to the U.S. State Department earlier this year stating that ISIS maintains a detailed store-like price list resembling a la carte food menu for selling captive females as sex slaves to Turkish, Syrian and Gulf Arab men, Breitbart reports. The 278-page investigative report came from the Knights of Columbus, the world's largest Catholic fraternal organisation, and the nonprofit In Defense of Christians.

The report noted that the terrorist organisation has admitted a "significant decrease" in the demand for sex slaves, who ISIS terms as "spoils of war," which adversely affects the financing of its war and terror campaign.

Because of this, the ISIS "Committee of Treasury" has set regulations on the selling women, with the penalty of death imposed on those found guilty of violating the rules.

According to the report, the prices of sex slaves depend on their age, with younger ones fetching higher prices. Christian or Yazidi girls aged 1 to 9 years have the highest price tag of $169.21 each, followed by girls aged 10 to 20, $126.91; women aged 20 to 30, $84.60; women aged 30 to 40, $63.45; and women aged 40 to 50, $42.30.

In its Country Reports on Human Rights Practices for 2015, the U.S. State Department noted that ISIS has developed a detailed bureaucracy for sex slavery, including sales contracts notarised by ISIS-run courts.

Since August 2014, ISIS has captured and enslaved more than 5,800 Yazidis, including more than 3,000 women, the report says, citing the Yazidi Affairs Directory as its source.

The report says ISIS executes captives who refuse to marry jihadis in Iraq and Syria who bought them.

In December 2014, the ISIS even released guidelines on how to capture, forcibly hold, and sexually abuse female slaves, according to the U.S. State Department.

The report says ISIS routinely forces Iraqi and Syrian girls to undergo virginity tests before trading them in "slave bazaars" and sending them to various Syrian provinces and other countries for sexual slavery.

The abducted sex slaves are subjected to various forms of sexual violence including gang rape, forced marriages, and coerced abortions, according to the report.