More than 200 mass graves containing thousands of bodies have been discovered in Iraqi regions formerly controlled by Islamic State extremists, the United Nations reported on Tuesday.

There may be many more sites besides the 202 that have been documented in the northern and eastern regions of Nineveh, Kirkuk, Salah al-Din and Anbar, the UN Assistance Mission for Iraq (UNAMI) and the UN Human Rights Office in Geneva said.

"These graves contain the remains of those mercilessly killed for not conforming to [Islamic State's] twisted ideology and rule, including ethnic and religious minorities," said Michelle Bachelet, the UN high commissioner for human rights.

Evidence gathered from these sites will be important for bringing the perpetrators to justice, the UN organizations said.

The smallest mass grave, in west Mosul, contained eight bodies, while what's believed to be the biggest, south of Mosul, may be filled with the remains of thousands of people.

Islamic State seized large areas of Iraq between mid-2014 and the end of 2017. Iraq declared victory over the militant group in December after a US-backed campaign of three years.

Islamic State's systematic human rights violations may amount to crimes against humanity and genocide, the UN organizations wrote.

Families find it very hard to find their missing relatives because of bureaucratic procedures that require them to report to five separate authorities, according to the report.