Iraq's Yazidi minority is facing what may amount to an "attempted genocide" at the hands of Islamist fighters, a senior UN rights official said Tuesday after visiting northern Iraq.

"The evidence strongly indicates attempt to commit genocide," UN Assistant Secretary General Ivan Simonovic said after meeting with officials and displaced people in Arbil, Baghdad and Dohuk during the week-long visit.

Simonovic told reporters at UN headquarters that the atrocities perpetrated by Islamic State fighters over the past four months may amount to war crimes and crimes against humanity.

In the case of the Yazidis, he said the killings could be qualified as an attempted genocide because there was evidence of an intent to exterminate them if they refuse to convert.

Tens of thousands of Yazidis have fled, fearing for their lives after being targeted for their religious beliefs.

The Islamic State boasted in a recent issue of its Dabiq magazine that it was selling Yazidi women and children as slaves and that Yazidis were singled out because of their unique customs.

Simonovic was able to interview at least 30 Yazidis who described harrowing accounts including the execution of a group of Yazidis who were rounded up in a school and refused to convert.

Thousands of Yazidis remained trapped on a mountain near their main hub of Sinjar for days in August, while others were massacred and the fate of hundreds of missing women and children remained unclear.

Human Rights Watch released a report earlier this month that said abducted Yazidi women were subjected to sexual assault and were being bought and sold by IS fighters.

"The systematic abduction and abuse of Yazidi civilians may amount to crimes against humanity," the New York-based watchdog said in a statement.