Those killed on Thursday and Friday had been rounded up near and in the city for use as human shields against attacks that are forcing ISIS out of the southern sections of Mosul, the source explained.

ISIS used a bulldozer to dump the corpses in a mass grave at the scene of the executions -- Mosul's defunct College of Agriculture in the north of the city, the intelligence source said.

The victims were all shot and some were children, said the source.

The United Nations earlier said it is "gravely worried" that ISIS has taken 550 families from villages around Mosul and is using them as human shields as Iraqi and Kurdish forces battle the terror group for control of Iraq's second-largest city.

Two hundred families from Samalia village and 350 families from Najafia were forced out Monday and taken to Mosul in what appears to be "an apparent policy by ISIS to prevent civilians escaping," Ravina Shamdasani, deputy spokeswoman for the UN Human Rights Office, told CNN.

Zeid Raad Al Hussein, UN High Commissioner for Human Rights, said his office had evidence of several instances since Monday where ISIS forced civilians to leave their homes in outlying villages and head to Mosul. It also had received reports that civilians suspected of disloyalty had been shot dead.

"We are gravely worried by reports that (ISIS) is using civilians in and around Mosul as human shields as the Iraqi forces advance, keeping civilians close to their offices or places where fighters are located, which may result in civilian casualties," Zeid said.

"There is a grave danger that (ISIS) fighters will not only use such vulnerable people as human shields but may opt to kill them rather than see them liberated," he said.

His office is examining reports that ISIS shot dead at least 40 civilians in a village outside Mosul.

Any ISIS gunmen who are captured or surrendered "should be held accountable in accordance with the law for any crimes they have committed," he said.

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