ISIS crimes must be documented and evidence collected to allow for prosecutions, human rights lawyer Amal Clooney has said.

The high-profile barrister called for international courts to act and punish the perpetrators in Iraq and Syria, according to the Guardian.

But in order for this to happen ISIS' massacres, murders, mass rapes, and child enslavement must be recorded as evidence, she said, joining UK foreign secretary Boris Johnson and other international figures who have made similar calls.

Clooney is working with Iraq's beleaguered Yazidi community who, along with Christians, have been targeted by ISIS fighters in what many call a genocide.

Girls as young as eight are sold at slave markets with many gang raped, she told a conference on Iraq and Syria. But the militants are leaving behind a 'bureaucracy of evil' with vital paperwork that could serve as evidence in upcoming trials, she said.

'All of this evidence is going to be lost if it's not collected soon,' Clooney warned. 'Mass graves are being contaminated as relatives dig for remains of their loved ones. Documents are not being gathered. Witnesses are being dispersed around the world. They are increasingly reluctant to speak about these cases.'

Although 'the UK government has been taking the lead', Clooney said little progress has been made towards collating evidence.

'I believe that the crimes committed by Isis in Iraq are some of the worst of our generation,' Clooney said. 'Medieval types of violence were being committed in which there were slave markets. This is a test not just for the Iraqi government, but for the United Nations and international law.'

In order for the International Criminal Court (ICC) to investigate a case, a resolution authorising it must first be passed by the United Nations Security Council.

But despite repeated calls for the UK to table a resolution as one of the Council's five permanent members, officials have refused insisting Russia, another permanent member, would veto an investigation into war crimes in Syria.