Women and girls from Iraq's Yazidi religious minority have told rights activists they were beaten and forced into sexual slavery by the Islamic State jihadist group, driving some to suicide.

IS militants have overrun swathes of Iraq since June, declared a cross-border caliphate also encompassing parts of neighbouring Syria and carried out a litany of abuses in both countries.

The group has targeted Yazidis and other minorities in northern Iraq in a campaign that rights group Amnesty International said in a report Tuesday amounted to ethnic cleansing, murdering civilians and enslaving others for a fate that some captives consider worse than death.

It said hundreds and possibly thousands of Yazidi women and girls had been forced to marry, sold or given to IS fighters or supporters.

"Many of those held as sexual slaves are children -- girls aged 14, 15 or even younger," said Donatella Rovera, Amnesty's senior crisis response adviser, who interviewed dozens of former captives.

A 19-year-old named Jilan committed suicide out of fear she would be raped, according to the Amnesty report entitled "Escape from Hell: Torture and Sexual Slavery in Islamic State Captivity in Iraq".

"One day we were given clothes that looked like dance costumes and were told to bathe and wear those clothes. Jilan killed herself in the bathroom," said a girl who was held with her but later escaped.

"She cut her wrists and hanged herself. She was very beautiful; I think she knew she was going to be taken away by a man and that is why she killed herself."

Another former captive told the rights group that she and her sister tried to kill themselves to escape forced marriage, but were stopped from doing so.

"The man who was holding us said that either we marry him and his brother or he would sell us," said Wafa, 27.

"At night we tried to strangle ourselves with our scarves. We tied the scarves around our necks and pulled away from each other as hard as we could, until I fainted," she said, but two other captives stopped them.

Sixteen-year-old Randa was abducted with her family, then beaten and raped by a man twice her age. Her male relatives were killed.

The man "took me as his wife by force. I told him I did not want to and tried to resist but he beat me. My nose was bleeding, I could not do anything to stop him," Randa said.

"It is so painful what they did to me and to my family," she said.

- IS boasts of abuse -

Amnesty said that many of the perpetrators were IS fighters, but might also include supporters of the group.

Some of the escaped victims said they were kept in family homes with wives, children, parents and siblings of the rapists.

IS has boasted in its propaganda magazine "Dabiq" of the horrors it has inflicted.

In an article entitled "The revival of slavery before the hour", Dabiq argues that by enslaving people it claims hold deviant religious beliefs, IS has restored an aspect of Islamic sharia law.

"After capture, the Yazidi women and children were then divided according to the sharia amongst the fighters of the Islamic State who participated in the Sinjar operations," the article said, referring to the area where the Yazidis were seized.

The abductions and rapes have drawn widespread international condemnation.

US Secretary of State John Kerry has denounced the enslavement of women and girls by IS as "abhorrent".

The abuse causes long-term damage even to those who manage to escape.

"The physical and psychological toll of the horrifying sexual violence these women have endured is catastrophic," Rovera said.

"Many of them have been tortured and treated as chattel. Even those who have managed to escape remain deeply traumatised."

One man said that he fears his wife, who escaped captivity, may commit suicide, and makes sure someone is with her at all times.

"My wife has panic attacks and can't sleep. I can't leave her alone because I'm afraid for her safety," he said.