The fate of hundreds if not thousands of women who have been kidnapped by Islamic State militants in northern Iraq and reportedly forced into marriage or raped is still largely unknown, according to United Nations and human rights officials.

Most of the missing women are members of Iraq's Yazidi minority who were captured in towns where locals were unable to flee quickly enough to Mt. Sinjar and Kurdish-controlled areas as ISIS launched its offensive in early August. The terror group is also believed to have abducted Shia Turkmen, Christians, and Shabak women.

Shortly after their capture, the UN cited reports of "barbaric" sexual violence and "savage rapes" perpetrated by ISIS.

Francesco Motta, director of the UN's human rights office in Iraq, told VICE News that the number of women captured in and around Nineveh Province in the northeast of Iraq could be as high 2,500 — including 1,000 who were forcibly converted and possibly married off, as well as 1,500 who refused to do so and languish in captivity.

"A number of women have been told that they will be taken to Syria or other neighboring countries and sold there," he said.

Over the past month, the UN received further reports from Mosul, where some of the women are allegedly being offered up "cheap" to young men as a way to attract them to join Islamic State forces. By dispersing the women with no oversight, it is feared that many of them will never be heard from again.

Human rights observers say that corroborating individual crimes and their circumstances is difficult given the near impossibility of accessing Islamic State territory. Several witness accounts have emerged, however.

Fred Abrahams, a special advisor to Human Rights Watch, was with Yazidi families who fled to Kurdish-controlled areas when they were able to make contact via cell phones with loved ones held by Islamic State militants.

One captive described being held with many "young girls in detention along with a number of women and children, and [militants] coming and taking away the young women," Abrahams recounted to VICE News. "Another woman got away from ISIS six days ago and told me they had come to the house where she was being held and taken away young women who are not married or didn't have children — who they didn't believe were married — then forcibly married them. They actually let them come back and say goodbye, then they were taken away."

Women and girls who had escaped captivity from the Islamic State gave similar accounts to Amnesty International, which reported that many women "had been removed from their places of detention and sent away to be forcibly married; they were told that if they refused they would be sold."

Abrahams had also heard credible reports of the open sale of women in Mosul.

Haleh Esfandiari, director of the Wilson Center's Middle East program, told VICE News that she'd heard reports that locals within the Islamic State had attempted to buy captive women from slave markets in order to free them, only to be rebuffed by militants who would only sell them to other fighters.

"It appears normal now that you would bring a group of these women into a square and try to sell them," Esfandiari told VICE News, referencing reports she had seen. "This was going on in Syria after ISIS took over towns and villages."

Unconfirmed accounts on social media also show purported Islamic State fighters discussing the women and referring to them as slaves.

Walked into a room, gave salam to everyone in the room to find out there was a yazidi slave girl there as well.. she replied to my salam :/ — Umm Anwar (@UmmAnwar_)August 28, 2014

Sometimes Islamic State fighters try to imitate the trappings of a real marriage, as if the women were not captives. In the account Abrahams described of women returning to say goodbye after their forced marriages, the witness said one of her fellow prisoners had been gifted a small amount of gold at the ceremony.

"It's a religious veneer of enslavement," Abrahams remarked.

Islamic State authorities attempt to validate rape and forced marriage by way of an antiquated literal interpretation of passages of the Qur'an that say women and children captured during battle are slaves. They cite stipulations that sexual relations may take place within a marriage or between a man and his malak yamiin, or "those possessed by one's right hand" — a euphemism for a slave.

"They believe that during war, normal prohibitions are lifted and sex with captives is permitted," said Motta. "Some women have reported that their teenage sons and daughters have been sexually assaulted by ISIL fighters, either when they were first detained or subsequently," he added, using an alternative name for the Islamic State. "A number of women have told us that they would prefer to commit suicide and kill their children than submit to what ISIL is doing."

Legal experts whom VICE News consulted described the Islamic State's selective interpretation of Qur'anic verses as directly contradicting centuries of Islamic law, the spirit of the Qur'an, and even the Prophet's own words as recorded in the Hadith.

Prior to the abolition of slavery in Muslim countries starting in the 19th Century, Islamic law had historically allowed the enslaving of non-Muslims captured during battle — but only when engaged in international conflict, not civil war. Islamic law traditionally had two sets of rules for people living within the Islamic empire, which grew rapidly in the 7th Century, and those outside of it. Conflict between the Islamic empire and other nations had distinctive precepts.

"There's a bright line, a distinction between international law and civil war taking place on Muslim territory," Mohamed Fadel, Canada Research Chair for the Law and Economics of Islamic Law at the University of Toronto, told VICE News. "Classical Islamic doctrine states that non-Muslims living among Muslims have all the legal protections that Muslims have, including protection from enslavement. ISIS is completely repudiating large areas of traditional jurisprudence."

On June 29, ISIS leader Abu Bakr al-Baghdadi proclaimed himself caliph of the rebranded Islamic State, with little backing from respected religious authorities and unease from many in the Sunni community. By positioning its self-proclaimed caliphate as the only true Muslim state, and thereby considering all other regions as outside that realm, the Islamic State considers non-Sunnis infidels.

Its handling of non-Sunnis varies — for instance, it's been said that Christians in some areas have been allowed to continue living while paying a tax — but the doctrine of accusing others of apostasy (those who do so are traditionally referred to as Takfiris) underpins the Islamic State's rationale for violence, said Fadel.

Yusuf al-Qaradawi, a prominent Egyptian Sunni religious scholar and an intellectual leader of the Muslim Brotherhood, called Baghdadi's declaration dangerous and void under Sharia law, explaining that the title of caliph can "only be given by the entire Muslim nation," not a single group.

"Takfir doctrine is so alarming — it's not simply a theological declaration that someone is an atheist or apostate," Fadel said. "Anyone who is not with them is not entitled to legal benefits of being a Muslim or non-Muslim living among Muslims."

In a speech on Monday, the UN's new human rights chief, former Jordanian Ambassador Prince Zeid Raad al-Hussein, called Islamic State leaders Takfiris.

"They reveal only what a Takfiri state would look like," he said. "It would be a harsh, mean-spirited house of blood, where no shade would be offered, nor shelter given, to any non-Takfiri in their midst. In the Takfiri world, unless your view is identical to theirs — and theirs is extremely narrow and unyielding — you forfeit your right to life."

Groups fixated on Takfir date to ancient Arabia. In the 88th book of the 16th Hadith, Sahl ibn Hunaif, a sahabi (disciple) of Muhammad, is asked if the Prophet spoke of the Khawarij, a deviant sect of religious zealots formed in the late 7th Century that attacked mainstream Muslims as non-believers who should be killed.

"I heard him saying while pointing his hand towards Iraq," recounted Hunaif. "There will appear in it (i.e. Iraq) some people who will recite the Qu'ran but it will not go beyond their throats, and they will go out from Islam as an arrow darts through the game's body."

"The Messenger of Allah said: 'The Khawarij are the dogs of Hell,' " another sahabi recounted.

For many Muslims, the discussion of a caliphate (the last caliphate, in Ottoman Turkey, was abolished in 1924) and questions over slavery are as arcane as they are frustrating. But the debate and global outcry have not affected their ability to attract foreign fighters, thousands of whom have streamed into Iraq and Syria.

The Islamic State's strict and self-serving citation of scripture derives most recently from the Wahhabist movement that dominates Saudi Arabia and proliferates through Saudi-funded madrassas that have spread throughout the Muslim world. The movement, which traces its roots to the 18th Century, insists on strict adherence to the Qur'an and Hadith, and does not recognize a millennium of interpretation and reconciliation of scripture and law with the accepted reality of contemporary society.

"Unfortunately, ISIS is one of the consequence of the puritanical literalism that has taken over and become one of the dominant trends in the Muslim and Arab world," said Fadel. "The Wahhabis are saying, 'If you would attempt to change any practice that existed in the time of Muhammad, you are essentially committing apostasy.' That is an extreme application of a doctrine that many people at first glance accept. And its application to slavery is particularly perverse."

Motta said the forced marriages taking place among women who converted to Sunni Islam in Islamic State-controlled territory could superficially be "normal marriages" under Islamic law, or they may be akin to another form of marriage that is sanctioned in Islamic jurisprudence — nikah misyar, or "traveler's marriage." That arrangement, which affords women fewer rights than a traditional marriage, can end without an official abrogation.

In any case, the marriage of captive women could be understood as void, since marriage in Islam requires the consent of both parties.

Nikah misyar has been criticized as a workaround for men to have sex with women outside of their marriage. However, like the Shia practice of temporary marriage called nikah mut'ah, it can have practical uses in conservative cultures, such as lessening the financial burden of marriage.

The practice is particularly popular in Gulf nations where Wahhabism is entrenched, according to Tariq al-Jamil, a professor of religion at Swarthmore College and an expert on medieval Islamic law.

"Salafis and Wahhabis have been attempting to come up with ways of making licit sex with whom one is not married," Jamil told VICE News. "These ideas have been floating around as a sort of hilah, or loophole, that can be exploited by people."

"Once one starts making things like misyar permissible, and being very literal in one's approach to reading and interpreting the text, one begins to see something as reprehensible as sexual slaves as okay," he added. "You stop asking the question, 'Is it okay to take people as slaves?' Wahhabism is about imperatives about what to do, not how to think about things or moral and ethical reasoning."

For nearly a half century, despite heavy Saudi involvement in extremism, United States foreign policy has largely kept mum on a country that is often characterized as one of the America's closest Arab allies. On Wednesday, President Barack Obama consulted with Saudi King Abdullah ahead of a much-anticipated speech on American military action against the Islamic State.

"What needs to be done is to develop some sort of short, medium, and long-term plan for recreating a sort of civil politics in the region," said Fadel, but he thinks such an endeavor would not be helped by the US relationship with Saudi Arabia. "The Gulf states resist even the most baby steps towards any sort of democratic accountability."

The suffering of women in Iraq is not unique to Islamic State-held territory, a fact that Amber Khan, a gender and human rights advocate at the non-profit group Women for Women, emphasized to VICE News.

"Our colleagues in Iraq have confirmed reports of the rise of gender-based attacks and violent atrocities," she said. "The attacks on women and girls have a chilling effect outside of the regions controlled by ISIS."

Khan noted that women's groups have faced greater intimidation and security threats of late.

"These are happening in areas near Baghdad, where ISIS is not in control, and represent how dangerous it has become for women," she said.

Violence against women is also not exclusively carried out by Sunni Islamist groups. On July 12, a group of men suspected of being part of a local Shia militia slaughtered 29 women in a suspected brothel in Baghdad, gunning them down as they cowered together in beds, showers, and on the floor.

"This is the destiny of all prostitutes," the men wrote on a wall.

Abrahams and others worry that the Yazidi community, still reeling from the Islamic State's invasion and slaughter, could regard its victimized women as shamed, and turn on them.

"The Yazidi community, like much of Iraq, has a tradition of honor killings," he said. "When these young women are released, as I hope they will be, those who have suffered sexual attacks or sexual violence could be susceptible to honor killings."

A concerted American strategy against the Islamic State is expected to unfold in the coming days, but human rights officials are concerned that the escalation of a bombing campaign and the prospect of a ground operation will prompt militants to use the women as hostages under threat of death.