Human rights lawyer Amal Clooney has urged the world not to let the Islamic State group "get away with genocide".
She said it was not enough to kill IS "on the battlefield", as perpetrators must be brought to justice in court.
Mrs Clooney, who represents victims of IS rapes and kidnappings, was speaking at the United Nations HQ in New York.
She said the "passive response" by the world's nations to a campaign to properly investigate the IS crimes was "shocking".
She urged Iraq's Prime Minister Haider al-Abadi to send a letter to the UN Security Council so it can vote to set up a probe into crimes into his country.
"Justice is what the victims want," Mrs Clooney said at the meeting, titled The Fight Against Impunity for Atrocities: Bringing Da'esh to Justice.
"But justice will be forever out of reach if we allow the evidence to disappear, if mass graves are not protected, if medical evidence is lost, if witnesses can no longer be traced," she said.
"Killing ISIS on the battlefield is not enough. We must kill the idea behind ISIS by exposing the brutality and bringing individual criminals to justice.
"Not one ISIS militant has faced trial for international crimes anywhere in the world. Why is it that nothing has been done?"
Mrs Clooney, who is married to Hollywood actor George Clooney, represents Nadia Murad, a Yazidi woman captured by IS in Iraq in 2014, who was also at the meeting.
Ms Murad, now a goodwill ambassador for the UN, has spoken about being raped, sold as a sex slave, and praying for death while in captivity.
"Why it is taking so long?" Ms Murad said, her voice breaking with emotion.
"I cannot understand why you are letting ISIS get away with it, or what more you need to hear before you will act.
"So today, I ask the Iraqi government and the UN to establish an investigation and give all the victims of ISIS the justice they deserve."