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Militants use the Facebook-owned messaging apps to share pictures from a database of girls held captive. The apps end-to-end encryption capabilities allow data to be shared privately – even prohibiting the developers from accessing messages from users.

An estimated 3,000 women and girls – many of which stem from the minority Yazidi community – are said to be held as sex-slaves by the group across Syria and Iraq.

"We condemn the inaction of social media websites such as Telegram, Facebook, Twitter and WhatsApp, for allowing the trade of the Yazidi women and children," Ahmed Burjus, the director of  in the UK, an organisation that was formed to support the Yazidi community in the aftermath of the 2014 genocide, told The Independent.

"Social media websites [such as] Twitter and Facebook have not responded to requests to remove materials such as auction pages [selling] Yazidi women.

"[IS] are also insulting Yazidi families everyday through these apps by sending pictures of their kidnapped members."

Matt Steinfeld, a spokesperson for WhatsApp, , "We have zero tolerance for this type of behaviour and disable accounts when provided with evidence of activity that violates our terms.

"We encourage people to use our reporting tools if they encounter this type of behaviour."

In March, Iraqi forces freed a group of Yazidi women held captive by the Islamic State group (IS) from the group's stronghold in Mosul.

Thousands of Yazidis were taken hostage by IS when the group overran huge swathes of Iraq in June 2014.

Survivors who managed to escape the radical group's captivity have spoken of abhorrent treatment and sexual violence.

Yazidis are a religious minority in Iraq viewed as heretics by the extremists. Islamic State has committed mass executions and enslavement against their community.