ISIS executes hundreds of Mosul area residents

(CNN) - ISIS executed 284 men and boys as coalition forces closed in on Mosul, an Iraqi intelligence source told CNN.

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, who wanted anonymity because he is not authorized to speak to the media. CNN could not independently confirm the claim.

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.

Meanwhile, ISIS militants attacked security buildings in Kirkuk, 175 kilometers (109 miles) southeast of Mosul.