Human Rights Watch (HRW) said on Thursday that ISIS militants probably killed over 300 Iraqi former police three weeks ago and buried them in a mass grave near the town of Hammam Al-Alil located south of Mosul.

Residents of the site of the mass graves said the militants buried the victims who had been shot or beheaded.

The residents told a Reuters reporter visiting the site that they believed up to 200 people were killed by the terrorist group in the weeks before ISIS withdrew from the town.

“This is another piece of evidence of the horrific mass murder by ISIS [Daesh] of former law enforcement officers in and around Mosul,” said Joe Stork, deputy Middle East director at Human Rights Watch.

“ISIS should be held accountable for these crimes against humanity,” he added.

Since the Mosul operation began, HRW has documented ISIS forcibly evacuating civilians under their rein with its militants, in order to use them as “human shields,” and executions of former Iraqi security forces.

ISIS has also conducted at least three chemical attacks on civilians south of Mosul.

Widespread or systematic killings conducted by a state or organized group as part of an attack on a civilian population constitutes a crime against humanity.

Also, the killing of prisoners, civilian or military, during armed conflicts constituted a war crime.