"It's the first mass grave of this kind to have been discovered" near Tal Afar, some 70 kilometres west of Mosul, local official Abdelaal Abbas said.
Iraqi forces retook Mosul from IS in July and Tal Afar in late August, three years after the jihadists overran the northern Iraqi cities.
"IS would throw the bodies of its fighters... in a deep pit seven kilometres north of Tal Afar," Abbas said.
Another security source, who spoke on condition of anonymity, said the bodies were those of "men and women, some probably Chechen or Turkish".
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"It's likely these IS members were killed several months ago during an air strike and not during the offensive of Iraqi forces to oust the jihadists from Tal Afar."
The names of the jihadists and when they were killed appeared on memorial plaques near the grave, the source added.
In Iraq, their presence has been reduced to the northern town of Hawija -- where Iraqi forces began an offensive today -- and a stretch of the Euphrates Valley near the border with Syria.
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