BAGHDAD — More than 200 mass graves have been uncovered in parts of Iraq formerly controlled by Islamic State fighters, according to the United Nations.

The graves contain the remains of thousands of people who are believed to be the victims of the Sunni militant group.

The dead include women, children, the elderly and disabled, as well as Iraqi soldiers and police officers, a U.N. report said Tuesday.

ISIS overran swaths of northern Iraq in 2014 and declared it part of their so-called caliphate. They were defeated by Iraqi forces supported by a U.S.-led coalition in December 2017.

The United Nations has said previously that almost 33,000 civilians were killed by the group in Iraq, with more than 55,000 injured.

The United Nations estimates that 6,000 to 12,000 bodies are contained in the 202 graves documented in the report.

But exact numbers were difficult to establish, it said, since only 28 mass graves have been excavated so far, with just 1,258 bodies exhumed.

The deaths occurred in what the United Nations has called a systematic and widespread campaign of violence, "which may amount to war crimes, crimes against humanity and genocide."

During its three-year rule, the group terrorized local residents, conducting well-publicized executions of people targeted for their perceived opposition, government ties, sexual orientation and more.

The militants also targeted members of ethnic and religious minorities, including Christians and Yazidis.

The smallest mass grave, found in Mosul, contains eight bodies. The biggest, the Al Khasfa sinkhole south of Mosul, is thought to contain roughly 4,000 bodies.

The United Nations said the sites must be protected from interference or contamination to ensure justice and accountability.

But the Iraqi government body tasked with preserving mass graves says it is underfunded and understaffed, and cannot adequately protect and investigate the sites, which are still littered with unexploded ordnance.

"Determining the circumstances surrounding the significant loss of life will be an important step in the mourning process for families and their journey to secure their rights to truth and justice," U.N. Representative for Iraq Jan Kubis said.