The UN Security Council on Thursday unanimously decided to set up an investigation team to collect evidence on the massacres of Iraq's Yazidi minority and other atrocities committed by the Islamic State group in Iraq.

Britain drafted the resolution to help bring perpetrators of IS war crimes to justice -- a cause championed by international human rights lawyer Amal Clooney, who was present for the vote.

The Lebanese-British lawyer represents Yazidi women who were taken hostage and used as sex slaves by IS as it swept into Iraq's Sinjar region in August 2014.

Clooney, the wife of actor George Clooney, sat next to Nadia Murad, a Yazidi survivor of IS enslavement, as the council voted on the measure.

"This resolution is a victory for victims of ISIS who have fought for so long for justice," Clooney said in a statement.

"Through its unanimous vote today the UN has sent a strong message that ISIS can no longer commit genocide, crimes against humanity and war crimes with impunity. And that victims may finally have their day in court."

The United Nations has described the massacres of the Yazidis as genocide and Clooney has over the past month made high-profile appearances before the world body to demand action.

After months of pressure Iraq in August agreed to the investigation, which will "support domestic efforts to hold" IS jihadists accountable by "collecting, preserving and storing evidence" in Iraq, the resolution said.

Under the measure, UN Secretary-General Antonio Guterres will within 60 days present to the council details on the mandate of the investigative panel that will work with its Iraqi counterparts.

- Major first step -

The investigators will gather evidence on "war crimes, crimes against humanity or genocide" for use in Iraqi courts that will hold trials for IS militants, according to the resolution.

US Ambassador Nikki Haley described the resolution as "a major first step" to help victims of IS crimes in Iraq.

IS fighters have been on the run in Iraq since the recapture of Mosul, Iraq's second city in July, which had been under IS rule since 2014.

Tens of thousands of Yazidis fled the August 2014 massacre in Sinjar, and UN rights investigations have documented horrific accounts of abuse suffered by women and girls.

Around 3,000 women are believed to remain in IS captivity.

After the vote, Murad expressed gratitude to the Security Council on behalf of Yazidi victims.

"I hope we can now move swiftly to create this team with a strong capacity and start exhuming mass graves and collect evidence," Murad said in a statement.

"Although ISIS is being defeated on the ground, victims need justice in order to allow healing and reconciliation and we hope this resolution will mark the beginning of that process."

Human Rights Watch criticized the resolution as a missed opportunity by the council to address atrocities committed by Iraqi and other forces.

"No one denies the importance of tackling the widespread atrocities by ISIS in Iraq, but ignoring abuses by Iraqi and international forces is not only flawed, it's shortsighted," said HRW's justice expert Balkees Jarrah.

The Iraqi government worked with Britain to draft the measure.