UN High Commissioner for Human Rights Navi Pillay said in a statement Monday that the atrocities committed by the group, which is active in Iraq and Syria, will amount to "war crimes and crimes against humanity".
"Grave, horrific human rights violations are being committed by the IS and associated armed groups on a daily basis," Pillay said.
"The violations include targeted killings, forced conversions, abductions, trafficking, slavery, sexual abuse, destruction of places of religious and cultural significance and the besieging of entire communities because of ethnic, religious or sectarian affiliation" the statement added.
It said among those directly targeted were Christians, Yazidis, Shabaks, Turkomen, Kakae and Sabaeans.
"Individuals who agreed to convert are being held under IS guard while men who refused to convert were executed," Pillay said.
Women and their children were taken as slaves, she said.
She added that "in Cotcho village in Southern Sinjar, IS killed and abducted hundreds of Yazidis Aug 15".
"Inhabitants of a number of other villages in Sinjar, which remains besieged by the IS and associated armed groups, are at serious risk," the statement said.
Pillay also condemned the June 10 "massacre" of about 650 of the 1,500 inmates in Mosul's Badoush Prison.
She expressed concern about the fate of some 13,000 members of the Shia Turkmen community in Saladin Governorate, including 10,000 women and children, who are besieged by IS and associated armed groups since June 15.
"The civilians are living under harsh conditions with severe food and water shortages and a complete absence of medical services. There are also fears of a possible imminent massacre", Pillay warned.