Western ISIS recruits are believed to be responsible for more than half the genocide crimes committed against the Yazidi people.
A report into the actions of ISIS foreign fighters reveals that dozens of Yazidi survivors encountered terrorist recruits in captivity.
The terrorist group massacred thousands of Yazidi men and older women in 2014 and left them in mass graves.
The militants then took young boys to train as child soldiers and girls as sex slaves.
The Counter Extremism Project interviewed 67 genocide survivors, many of whom said they were abused or assaulted by western ISIS recruits.
In one case, a Yazidi woman describes the involvement of British extremist Siddhartha Dhar, a former bouncy castle salesman and ISIS convert from east London.
She said that Dhar, who is also known as Abu Rumaysah, kept her captive and raped her as a 16-year-old.
The report's author, CEP strategic adviser Liam Duffy, said eyewitness accounts collected from survivors demonstrate "widespread involvement" of western ISIS recruits in the enslavement of Yazidis.
"At the very least, the involvement of westerners in the mass rape and enslavement of Yazidi women and children was more widespread than public perceptions or prosecution rates would suggest," he said.
"Of 67 survivors surveyed, only 10 had not come into contact with foreign members, meaning some 85 per cent had come into contact with foreign ISIS members during their time in captivity.
"Just over 50 per cent of respondents had come into contact with western ISIS recruits, from western Europe, North America or Australia, while in captivity."
Using archived social media activity, interviews with captured or stranded foreign fighters and first-hand testimonies from those held captive by ISIS, the report collated evidence of western foreign fighters’ "extensive involvement" in the atrocities.
Mr Duffy called on European governments to take action against those involved in war crimes.
“As the fate of hundreds of their own citizens who joined ISIS remains in the balance in northern Syria, western governments must investigate whether crimes against humanity were committed by these recruits, as well as those who have already returned to no sanction," he said.
"As this paper shows, involvement by westerners in the sexual enslavement of Yazidi women and girls was widespread, yet so far, no one has been made to answer for these atrocities.
"It is a moral and legal imperative that states committed to justice and the rule of law pursue accountability for the Yazidi genocide, starting with their own citizens.”
Germany is leading the way with trials seeking prosecutions for war crimes.
"Despite the widespread involvement of westerners in the genocide exemplified by this report, only Germany has brought a handful of charges against western home-grown recruits," Mr Duffy said.
"Many western ISIS recruits have even returned to their home countries to continue to live as civilians."
David Ibsen, executive director for CEP, called for ISIS fighters to be held accountable.
"The West has a responsibility to its citizens and to the Yazidi people, to hold these foreign fighters accountable for their heinous actions," he said.
"From a human rights and national security perspective, it is imperative that investigations are led into returnees and nationals stranded overseas for the crimes committed."
The UN's ISIS war crimes investigation team (Unitad) is hoping to launch prosecutions this year using evidence uncovered in the mass graves.
This month, the Iraqi parliament approved a law that for the first time recognises the crimes committed by ISIS against Yazidi women and children, and other ethnic and religious groups, as genocide and crimes against humanity.
