(CNN)ISIS executed 284 men and boys as coalition forces closed in on Mosul, an Iraqi intelligence source told CNN.
Source: ISIS executes hundreds of Mosul area residents
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.
• The UN human rights chief cites reports that civilians have been shot dead by ISIS.
• Nearly 30 ISIS militants have taken over a building in southern Kirkuk and fired on security forces.
• Twelve people were killed in a separate ISIS attack in Dibis.
• Iraqi-led forces have recaptured at least 100 square kilometers (39 square miles) of territory, CNN analysis shows.
Zeid Ra'ad 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 fighters who are captured or surrendered "should be held accountable in accordance with the law for any crimes they have committed," he said.
Dozens of militants targeted four police stations and Kurdish security offices in Kirkuk, spreading themselves out through neighborhoods.
Security forces killed at least seven ISIS militants in Kirkuk, officials there said, but there was no information yet on civilian casualties. Images broadcast on local television showed what appeared to be dead or injured fighters on the street.
Nearly 30 ISIS militants took over an unoccupied building in Domiz district in southern Kirkuk and started firing on security forces there, according to security officials. Iraqi security forces are surrounding the building now, and sporadic clashes continue.
The situation remains tense in the city, with at least two ISIS bombers hiding inside two buildings in southern Kirkuk, security officials said. Security forces surrounded both locations.
Local authorities imposed a curfew in Kirkuk.
The media wing of ISIS, Amaq, said online that ISIS fighters had attacked Kirkuk before dawn and taken control of 10 neighborhoods as well as carried out attacks to the north and south of the city.
Previous attacks by ISIS militants on Kirkuk have been attempts either to capture the city from the Peshmerga, as the Kurdish fighters are known, or divert Kurdish troops from the fight in Mosul.
In a separate incident, ISIS militants also attacked a government building in Dibis, about 40 kilometers (25 miles) northwest of Kirkuk.
Twelve people were killed, including nine Iraqi employees and three Iranian contractors, two security officials told CNN.
The most likely reason for Friday's assault in Kirkuk is disruption, with ISIS demonstrating it can deploy resources far behind the front lines, CNN's Michael Holmes said near Mosul.
"It's long been thought that there would be something like this going on in more than one place around Iraq as the Mosul offensive got under way," Holmes said. "There's been speculation that there are ISIS sleeper cells, or ISIS fighters, within reach of places like that for some time, from Baghdad to places like Kirkuk."
The city's significance stems its large oil reserves, which are almost as much as those in southern Iraq.
Iraqi planes have dropped more than eight million leaflets over ISIS-controlled areas of Anbar, Kirkuk and Nineveh provinces, the Iraqi Joint Operations Command said. The fliers ask residents to call in tips to a toll-free phone number.
On Thursday, fighting outside of Mosul as part of the offensive to retake ISIS' last bastion of power in Iraq was the fiercest yet, with Iraqi-led forces meeting strong resistance from militants.
Iraqi Maj. Gen. Maan al-Saadi said 200 ISIS fighters were killed as Iraqi forces captured the Christian town of Bartella, the latest win for the coalition of around 100,000 closing in on Mosul.