A single ISIS militant is suspected to be alone responsible for gunning down at least 150 women and girls, including some who were pregnant, because they refused to marry jihadists.
Abu Anas Al-Libi executed the women before burying their bodies in mass graves in Fallujah in the Iraqi province of Al Anbar, the Iraqi Ministry of Human Rights said in a statement.
It is thought he killed the women, most of whom were Yazidi, because they refused to enter into sham temporary marriages with Islamic State fighters, where they would effectively be treated as sex slaves.
'At least 150 females, including pregnant women, were executed in Fallujah by a militant named Abu Anas Al-Libi after they refused to accept jihad marriage,' the statement said.
'Many families were also forced to migrate from the province's northern town of Al-Wafa after hundreds of residents received death threats.'
The statement went on to say that the militants then turned a mosque in Fallujah into a prison, where they held hundreds of men and women.
Al- Libi is not the extremist with the same name who is alleged to have helped carry out East Africa's embassy bombings in 1998 that killed 224 people in Kenya and Tanzania.
ISIS fighters have taken thousands of women and children as captives, especially around 2,500 women from the minority Yazidi sect in Iraq.
A UN report last month said these women were then taken as far as Raqqa in Syria and sold to ISIS fighters, or given as gifts to their leaders. Some of them, it claims, were then used for sex.
It was recently revealed that ISIS has published a shocking guidebook for its fighters on how to rape women and girls they had taken into slavery.
The Arabic manual, titled Questions And Answers On Taking Captives And Slaves, instructs ISIS fighters on how to buy and sell women and girls who have been captured in war as booty.
It was recently revealed that ISIS has published a shocking guidebook for its fighters on how to rape women and girls they had taken into slavery
The document, published by the Research and Fatwa Department of ISIS, gives its fighters the green light to turn captured women into slaves and concubines, and even give them as gifts to one another.
It was circulated on the Twitter accounts of senior IS leaders and was distributed by masked IS fighters outside a large mosque in Iraq's second city Mosul, which is controlled by the group.
The document says that all 'unbelieving' women, including Jews and Christians, can be taken as captives and sold as slaves. However, it prohibits the enslavement of Muslim women, even if they have become apostates.
ISIS controls parts of Iraq, as well as large swathes of northern and eastern Syria.
This week, the bodies of 230 people killed by the militant group were found in a mass grave uncovered by their relatives in Syria's Deir Ezzor province.
The discovery brings the number of Shaitat tribal members slain during the jihadists' summer advance in Deir Ezzor province near Iraq to more than 900, the Syrian Observatory for Human Rights said.
The Britain-based group said it had 'learned from trusted sources that more than 230 bodies have been found in a mass grave in the desert near Al-Kashkiyeh in the east of Deir Ezzor'.
The 'vast majority' were civilians, many of them executed in cold blood after the tribe rose up against the Islamic State after it had driven out rival jihadists and rebels from the area.