"Bangura and Mladenov said they have received atrocious accounts on the abduction and detention of Yazidi, Christian, Turkomen and Shabak women, girls and boys, and reports of savage rapes," UN spokesman Stephane Dujarric said.
"Some 1,500 Yazidis and Christians may have been forced into sexual slavery," Xinhua quoted Dujarric as saying.
Mladenov called on regional governments and the wider international community for the immediate release of the women and girls held in captivity.
Tens of thousands of Iraqi refugees fled into the mountains, perhaps hoping to reach the Kurdish region in north Iraq, but were trapped because of militant activity between the mountain and the Kurdish area, and are running short of food and water.
The World Health Organisation (WHO) Wednesday said thousands of people who are still trapped in Sinjar Mountain are in immediate need of humanitarian aid.
So far, the WHO has deployed two mobile medical teams to Sinjar mountain and they would be stationed there until the evacuation of all displaced people from the mountain.