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World-renowned human rights lawyer Amal Clooney called for the collection of evidence of Islamic State [IS] group atrocities, which she described as "the worst crimes of our generation" during a presentation at Chatham House in London on Wednesday.

Clooney, who represents Yazidi women who escaped from IS enslavement, spoke alongside UNHCR's UN high commissioner on human rights, Zeid Raad al-Hussein at the event on accountability of crimes committed in Syria and Iraq.

The Lebanese-British lawyer warned evidence currently available against IS may be lost if undocumented.

"All of this evidence is going to be lost if it’s not collected soon," she said, according to newspaper.

"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."

Britain is leading a push for accountability for the crimes of IS - who seized control of large parts of Syria and Iraq in mid-2014 - declaring its Islamic caliphate and committing widespread atrocities.

"The UK government has been taking the lead," Clooney said.

"They have been making it a priority to launch an investigation and have drafted a resolution."

Earlier this month, Clooney urged Baghdad to agree to a UN investigation over the atrocities committed by IS.

"Why is it that nothing has been done?" she said, "Mass graves lie unprotected and exhumed. Witnesses are fleeing and not one [IS] militant has faced trial for international crimes anywhere in the world," she said.

Clooney said a British-drafted resolution setting up the investigation was ready to be submitted to the Security Council, but the Iraqi government has not sent a letter authorising it.