UN investigators say Islamic State (IS) extremists are committing genocide in Syria and Iraq against the Yazidi religious minority.

"Genocide has occurred and is ongoing," said Paulo Pinheiro, chairman of the UN Commission of Inquiry on Syria, in Geneva on June 16.

The special UN commission said in a report issued the same day that more than 3,200 Yazidi women and children -- taken from Iraq -- were still being held in Syria by IS fighters and used as sex slaves.

The report adds that IS militants carried out mass executions of Yazidis when they were based in Iraq in August 2014.

The report says sexual slavery, slave labor, torture, and forced conversions to Islam are the main methods used in the Yazidi genocide.

"Girls as young as nine were raped, as were pregnant women," says the report, largely based on interviews with survivors, activists, and medical personnel.

The report claims that Yazidi women were also prevented from giving birth by separating the men from the women and traumatizing females.

Commission investigators said the UN Security Council should take steps to stop the genocide and other crimes being committed against Yazidis.

There were an estimated 720,000 Yazidis in Iraq and Syria before the IS attacked the group on 2014.

Based on reporting by dpa and AFP