UN Secretary-General Antonio Guterres told the Security Council in the letter sent August 17 that the investigators led by British human rights lawyer Karim Asad Ahmad Khan would begin work on August 20.

The council last year unanimously adopted a resolution setting up the probe to help bring perpetrators of IS crimes to justice -- a cause championed by international human rights lawyer Amal Clooney.

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.

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

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.

Guterres told the council that Khan, who was appointed in February, had made a first mission to Iraq from August 6 to 14.

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.