As the Islamic State loses territory and retreats, it leaves behind grim evidence of its occupation: mass graves, filled with dozens or hundreds of bodies.

The Associated Press has documented 72 such graves in Iraq and Syria — and the wire service says many more are expected to be revealed as the Islamic State continues to cede ground.

Some of the gravesites are too dangerous to be excavated, according to the AP, and for at least 16 of them, officials won't even hazard a guess as to how many bodies are contained within.

"Still, even the known numbers of victims buried are staggering," the news service writes: "from 5,200 to more than 15,000."

Five of the mass graves are located at the foot of Mount Sinjar in Iraq, where the Islamic State made a concerted attack on a group of Yazidis — a religious and ethnic minority group — in 2014.

One young man, Arkan Qassem, told the AP he was watching through binoculars as Islamic State militants killed his friends and neighbors in Harden, Iraq, and buried them with bulldozers. Another man, Rasho Qassim, drives every day past the grave where his two sons are interred.

The AP reports:

"The sites are roped off and awaiting the money and the political will for excavation. The evidence they contain is scoured by wind and baked by sun." 'We want to take them out of here. There are only bones left. But they said "No, they have to stay there, a committee will come and exhume them later," ' said Qassim, standing at the flimsy protective fence."IS made no attempt to hide its atrocities. But proving what United Nations officials and others have described as an ongoing genocide will be complicated as the graves deteriorate. ..." 'There's been virtually no effort to systematically document the crimes perpetrated, to preserve the evidence,' said Naomi Kikoler, who recently visited for the Holocaust Museum in Washington, D.C."

The Yazidi graves are "a fraction" of the mass graves that have been left across former Islamic State territory, the AP writes.

One mass grave in Iraq is believed to include the bodies of some 600 inmates massacred at a prison in 2014. Another held 400 members of the Shueitit tribe; up to a thousand members of the tribe are believed to have been killed by ISIS.

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