WASHINGTON (Reuters) - The Islamic State is using several forms of contraception to maintain its supply of sex slaves, the New York Times reported on Saturday, citing interviews with more than three dozen Yazidi women who escaped from the militant group.

The New York Times reported that Islamic State used "oral and injectable contraception, and sometimes both" to ensure that the women did not become pregnant and could be passed among the fighters.

"In at least one case, a woman was forced to have an abortion in order to make her available for sex, and others were pressured to do so," the paper said.

Islamic State militants consider the Yazidis to be devil-worshippers. The Yazidi faith has elements of Christianity, Zoroastrianism and Islam. Most of the Yazidi population, numbering around half a million, remains displaced in camps inside the autonomous entity in Iraq's north known as Kurdistan.

Until late last year, some 5,000 Yazidi men and women were captured by the militants in the summer of 2014. Of those, around 2,000 had managed to escape or been smuggled out of Islamic State's self-proclaimed caliphate, activists said.

The New York Times, citing a gynecologist who carried out the examinations, said that out of the more than 700 Yazidi rape victims who had gone to a United Nations-backed clinic in Iraq, only 5 percent had become pregnant during their enslavement.

Dr. Nezar Ismet Taib, head of the Ministry of Health Directorate in Dohuk which oversees the clinic, said that number was much lower than expected, according to the newspaper.

The United Nations and human rights groups have accused the Islamic State of the systematic abduction and rape of thousands of women and girls as young as 12. Many have been given to fighters as a reward or sold as sex slaves.

Far from trying to conceal the practice, Islamic State has boasted about it and established a department of "war spoils" to manage slavery. Reuters reported on the existence of the department in December.

(This version of the story corrects in last line to "December" from "Monday")

(Reporting by Idrees Ali; Editing by Diane Craft)