(CNN) ISIS rounded up and killed 284 men and boys as Iraqi-led coalition forces closed in on Mosul , the terror group's last major stronghold in Iraq, an Iraqi intelligence source told CNN.

Those killed Thursday and Friday were used as human shields against attacks forcing ISIS out of southern parts of Mosul, the source said.

ISIS dumped the corpses in a mass grave at the defunct College of Agriculture in northern Mosul, the intelligence source said.

The victims -- including children -- were all shot, said the source, who asked for anonymity because he is not authorized to speak to the media. CNN could not independently confirm the killings.

Latest developments

US Defense Secretary Ash Carter arrived in Erbil on Sunday, after commenting that the Mosul operation was going to plan

Peshmerga forces started an offensive from two fronts in Bashiqa, northeast of Mosul, the Peshmerga General Command said Sunday

Prime Minister Abadi reiterated his view that Iraq does not need Turkey's assistance in the battle

Kurdish officials accused Sunni Arabs in Kirkuk of supporting ISIS a day after attacks killed dozens

Iraqis displaced by the Mosul offensive are seeking shelter in camps, the UN refugee agency said

Freeing a Christian town

The Iraqi military intensified its offensive Saturday to retake Hamdaniya -- also known as Qaraqosh -- from ISIS, the Iraqi Joint Operations Command center said. Iraqi forces and a Christian paramilitary group entered the town Wednesday, but face fierce resistance from ISIS fighters, 200 of whom were killed in one day of clashes.

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The city is about 15 kilometers (nine miles) southeast of Mosul.

Iraqi security forces and Peshmerga -- as the Kurdish fighters are known -- have made progress and isolated Hamdaniya, a US military official said in Baghdad, speaking on background.

Iraqi troops on Saturday entered the al-Askary neighborhood and liberated the mayor's building and the main hospital, raising the Iraqi flag over those buildings, Lt. Gen. Qassim al-Maliky said.

At least 60 ISIS fighters have been killed since Friday when Iraqi security forces launched an attack on the oil-rich city of Kirkuk, Gov. Najmaldin Karim said.

"Life will be restored to normal in the next few hours" Karim said Sunday, adding that security checkpoints in the city would be reopened. He did not mention how many security forces and civilians were killed in the ISIS offensive.

Tal Kayf is next target

Iraqi troops are also advancing toward Tal Kayf and plan on storming the Chaldean town, the Iraqi Joint Operations Command said Saturday.

Iraqi army and militia forces arrive Thursday in Saleh village in the offensive to wrest Mosul from ISIS.

Tal Kayf is about 10 kilometers (six miles) north of Mosul.

It's the closest Iraqi security forces have come to Mosul, a CNN analysis indicates.

The US military official said US and coalition aircraft were providing air support as needed Saturday. The official said land forces were working through "a hard outer crust," and resistance would intensify as the offensive neared Mosul.

The official said ISIS fighters have infiltrated towns cleared earlier, including Bartella, requiring renewed efforts to combat them.

Photos: Mosul: Iraqi-led forces push into key city Members of the Iraqi federal police wave the country's flag as they celebrate in the Old City of Mosul on July 9, 2017. Iraq declared victory against ISIS forces in Mosul after a grueling monthslong campaign. The battle to reclaim Mosul, the last major ISIS stronghold in Iraq, has been underway since fall 2016. Hide Caption 1 of 73 Photos: Mosul: Iraqi-led forces push into key city This injured girl was found by Iraqi forces as they advanced against ISIS militants in the Old City of Mosul on Monday, July 3. She was carried away for medical assistance. Hide Caption 2 of 73 Photos: Mosul: Iraqi-led forces push into key city A suspected ISIS fighter is held in a basement while Iraqi forces continue to push for control of the Old City of Mosul on Monday, July 3. Hide Caption 3 of 73 Photos: Mosul: Iraqi-led forces push into key city An Iraqi Special Forces soldier exchanges fire with ISIS militants in the Old City on Friday, June 30. Hide Caption 4 of 73 Photos: Mosul: Iraqi-led forces push into key city A bomb explodes near the al-Nuri mosque complex on Thursday, June 29. Iraq's military has seized the remains of the Great Mosque of al-Nuri. Iraq and the United States have accused ISIS of blowing up the historic mosque. Hide Caption 5 of 73 Photos: Mosul: Iraqi-led forces push into key city Members of the Iraqi Federal Police hold a position as US-led coalition forces advance through the Old City on Wednesday, June 28. Hide Caption 6 of 73 Photos: Mosul: Iraqi-led forces push into key city The remnants of Mosul's ancient leaning minaret are seen in the Old City on Sunday, June 25. ISIS' claim that US warplanes were responsible for the destruction of the minaret is "1,000% false," US officials told CNN. Hide Caption 7 of 73 Photos: Mosul: Iraqi-led forces push into key city Two boys comfort each other after their home collapsed during fighting between Iraqi forces and militants in Mosul on Saturday, June 24. The boys, who are cousins, said some of their relatives were still under the rubble. Hide Caption 8 of 73 Photos: Mosul: Iraqi-led forces push into key city An Iraqi soldier helps transport a girl as residents flee their homes west of Mosul on Friday, May 26. Hide Caption 9 of 73 Photos: Mosul: Iraqi-led forces push into key city Displaced Iraqis make their way through rubble after evacuating their homes in a neighborhood of west Mosul on Wednesday, May 17. Hide Caption 10 of 73 Photos: Mosul: Iraqi-led forces push into key city An Iraqi man tries to extinguish a burning car during fighting in Mosul's western Rifai neighborhood on Tuesday, May 16. Hide Caption 11 of 73 Photos: Mosul: Iraqi-led forces push into key city A member of the Iraqi counterterrorism service secures a building as troops push toward Mosul's Al-Oraibi western district on Sunday, May 14. Hide Caption 12 of 73 Photos: Mosul: Iraqi-led forces push into key city A smoke cloud rises on the front line as the Iraqi Emergency Response Division advances in west Mosul on Saturday, May 6. Hide Caption 13 of 73 Photos: Mosul: Iraqi-led forces push into key city A wounded man is transported in western Mosul on Friday, April 21. Hide Caption 14 of 73 Photos: Mosul: Iraqi-led forces push into key city An Iraqi federal policeman smokes during a break from battle on Wednesday, April 12. Hide Caption 15 of 73 Photos: Mosul: Iraqi-led forces push into key city A member of Iraq's security forces stands guard in eastern Mosul as smoke rises from the ISIS-controlled western section of the city on Friday, April 7. Hide Caption 16 of 73 Photos: Mosul: Iraqi-led forces push into key city Iraqis visit a bath house on the southern outskirts of Mosul on Wednesday, April 5. Hide Caption 17 of 73 Photos: Mosul: Iraqi-led forces push into key city Flames billow from an explosion in Mosul during a clash between Iraqi forces and ISIS fighters on Sunday, March 5. Hide Caption 18 of 73 Photos: Mosul: Iraqi-led forces push into key city Families are forced to evacuate as Iraqi forces advance in western Mosul on Thursday, March 2. The number of internally displaced people has surged as the offensive effort has intensified. Hide Caption 19 of 73 Photos: Mosul: Iraqi-led forces push into key city Mosul residents cross a damaged bridge in the al-Sukkar neighborhood on Saturday, January 21. Hide Caption 20 of 73 Photos: Mosul: Iraqi-led forces push into key city French President Francois Hollande and French Defense Minister Jean-Yves Le Drian, right, view territory held by ISIS during a visit to a military outpost near Mosul on Monday, January 2. Hide Caption 21 of 73 Photos: Mosul: Iraqi-led forces push into key city A young girl takes part in a Christmas Day Mass at a church in the predominantly Christian town of Qaraqosh. The area's churches were heavily damaged by ISIS militants before the town was freed by Iraqi forces during the Mosul offensive. Hide Caption 22 of 73 Photos: Mosul: Iraqi-led forces push into key city Iraqi Shiite fighters ride through a desert area near the village of Al-Boutha al-Sharqiyah, west of Mosul, on Friday, December 2. Hide Caption 23 of 73 Photos: Mosul: Iraqi-led forces push into key city Internally displaced Iraqis who fled the fighting in Mosul watch as a civilian drone films them at the al-Khazir camp on Thursday, December 1. Hide Caption 24 of 73 Photos: Mosul: Iraqi-led forces push into key city An Iraqi soldier searches a home for ISIS militants after Iraqi forces retook the village of Al-Qasr on Wednesday, November 30. Hide Caption 25 of 73 Photos: Mosul: Iraqi-led forces push into key city Iraqi soldiers transport a comrade who was injured during a battle near the village of Haj Ali on Tuesday, November 29. Hide Caption 26 of 73 Photos: Mosul: Iraqi-led forces push into key city A man mourns during the funeral of four Iraqi paramilitary fighters who were killed in battles in the town of Tal Afar. Hide Caption 27 of 73 Photos: Mosul: Iraqi-led forces push into key city Displaced civilians return to the village of Tall Abtah on Friday, November 25, after Iraqi forces retook the village from ISIS. Hide Caption 28 of 73 Photos: Mosul: Iraqi-led forces push into key city Iraqi civilians sit on the ground in Mosul on November 24. An Iraqi officer addressed the group, demanding to know the whereabouts of alleged ISIS militants who opened fire on troops a few days earlier. Hide Caption 29 of 73 Photos: Mosul: Iraqi-led forces push into key city An injured baby receives treatment at a field hospital in Mosul on November 15. Hide Caption 30 of 73 Photos: Mosul: Iraqi-led forces push into key city A woman cries Sunday, November 13, after seeing the St. Addai church that was damaged by ISIS fighters during their occupation of the Keramlis village. Hide Caption 31 of 73 Photos: Mosul: Iraqi-led forces push into key city An Iraqi special forces soldier prays next to a Humvee before troops pushed toward Mosul's Karkukli neighborhood on November 13. Hide Caption 32 of 73 Photos: Mosul: Iraqi-led forces push into key city A Kurdish Peshmerga fighter holds part of a defused bomb planted by ISIS militants in Bashiqa, Iraq, on Friday, November 11. Hide Caption 33 of 73 Photos: Mosul: Iraqi-led forces push into key city A member of Iraq's special forces guards two suspected ISIS fighters found hiding in a house in Mosul on November 11. Hide Caption 34 of 73 Photos: Mosul: Iraqi-led forces push into key city An Iraqi woman displaced by war holds her cat near a checkpoint in the Iraqi village of Shaqouli, east of Mosul, on November 10. Hide Caption 35 of 73 Photos: Mosul: Iraqi-led forces push into key city Iraqi troops watch a broadcast of Donald Trump's acceptance speech in a house in Arbid, on the outskirts of Mosul, on Wednesday, November 9. Iraqi Prime Minister Haider al-Abadi congratulated Trump on his win and said he hoped for continued support in the war on ISIS. Hide Caption 36 of 73 Photos: Mosul: Iraqi-led forces push into key city US Marines install equipment at a coalition base in Qayyara on November 9. Hide Caption 37 of 73 Photos: Mosul: Iraqi-led forces push into key city An Iraqi forces member investigates a mass grave that was discovered after coalition forces recaptured the area of Hamam al-Alil on Monday, November 7. Hide Caption 38 of 73 Photos: Mosul: Iraqi-led forces push into key city Iraqi children witness a man being interrogated by a member of the Iraqi army at a base next to the Al-Intissar neighborhood of Mosul on November 7. Hide Caption 39 of 73 Photos: Mosul: Iraqi-led forces push into key city A civilian man who fled the fighting trims his beard after reaching an Iraqi army position in Mosul on November 7. Hide Caption 40 of 73 Photos: Mosul: Iraqi-led forces push into key city Children play in debris created by an airstrike in Qayyara on Sunday, November 6. Hide Caption 41 of 73 Photos: Mosul: Iraqi-led forces push into key city Female members of the Freedom Party of Kurdistan sing as they hold a position near Bashiqa on November 6. Hide Caption 42 of 73 Photos: Mosul: Iraqi-led forces push into key city A baby is passed through a fence back to his mother at a refugee camp in the Khazir region on Saturday, November 5. Hide Caption 43 of 73 Photos: Mosul: Iraqi-led forces push into key city People line up to receive food at a refugee camp in the Khazir region on November 5. Thousands are taking refuge in camps set up for internally displaced people. Hide Caption 44 of 73 Photos: Mosul: Iraqi-led forces push into key city Iraqi soldiers pass near a bridge destroyed in an airstrike in Qayyara on November 5. Hide Caption 45 of 73 Photos: Mosul: Iraqi-led forces push into key city Iraqi soldiers patrol an alley on the outskirts of Mosul on Friday, November 4. Hide Caption 46 of 73 Photos: Mosul: Iraqi-led forces push into key city A suspected member of ISIS is detained at a checkpoint near Bartella, Iraq, on November 4. Hide Caption 47 of 73 Photos: Mosul: Iraqi-led forces push into key city Iraqi families pack into a truck to be moved to camps on Thursday, November 3. Hide Caption 48 of 73 Photos: Mosul: Iraqi-led forces push into key city An Iraqi special forces soldier searches for the location of an ISIS sniper in Gogjali on November 1. Hide Caption 49 of 73 Photos: Mosul: Iraqi-led forces push into key city A man fleeing the village of Bazwaya carries a white flag as he arrives at a checkpoint on November 1. Hide Caption 50 of 73 Photos: Mosul: Iraqi-led forces push into key city An Iraqi soldier receives treatment after being injured during clashes with ISIS fighters near Bazwaya on October 31. Hide Caption 51 of 73 Photos: Mosul: Iraqi-led forces push into key city An Iraqi soldier navigates through a shattered windshield as coalition forces advance on Bazwaya on October 31. Hide Caption 52 of 73 Photos: Mosul: Iraqi-led forces push into key city Archbishop Yohanna Petros Mouche, center, performs Mass in the liberated town of Qaraqosh on Sunday, October 30. Hide Caption 53 of 73 Photos: Mosul: Iraqi-led forces push into key city Children play in a camp for internally displaced people near Kirkuk, Iraq, on October 30. More than 600 families from Tel Afar, a town west of Mosul, have been living in the camp since ISIS took control of the area in 2014. Hide Caption 54 of 73 Photos: Mosul: Iraqi-led forces push into key city Displaced families are seen on the road near Qayyara on Saturday, October 29. Hide Caption 55 of 73 Photos: Mosul: Iraqi-led forces push into key city U.S. military personnel take cover in a bunker after a mortar alarm was sounded at a coalition air base in Qayyara on Friday, October 28. Hide Caption 56 of 73 Photos: Mosul: Iraqi-led forces push into key city Women and children grieve over the grave of a family member at a Qayyara cemetery damaged by ISIS on October 27. Hide Caption 57 of 73 Photos: Mosul: Iraqi-led forces push into key city Residents of Qayyara wait for distribution of food and water rations on October 26. Local water sources have been contaminated by the burning oil and sulfur. Hide Caption 58 of 73 Photos: Mosul: Iraqi-led forces push into key city Iraq's counterterrorism forces advance toward ISIS positions in Tob Zawa on October 25. Hide Caption 59 of 73 Photos: Mosul: Iraqi-led forces push into key city Soldiers give first aid to an injured boy in Tob Zawa on October 25. Hide Caption 60 of 73 Photos: Mosul: Iraqi-led forces push into key city Kurdish Peshmerga forces take positions as they start to move toward the Imam Reza and Tizxirab villages of the Bashiqa district on Sunday, October 23. Hide Caption 61 of 73 Photos: Mosul: Iraqi-led forces push into key city Iraqi forces distribute fruit in the village of al-Khuwayn, south of Mosul, after recapturing it from ISIS on October 23. Hide Caption 62 of 73 Photos: Mosul: Iraqi-led forces push into key city Kurdish security forces detain a suspected member of ISIS in the eastern suburbs of Kirkuk on Saturday, October 22. Hide Caption 63 of 73 Photos: Mosul: Iraqi-led forces push into key city An Iraqi forces member helps a man push a car as they arrive at a refugee camp in Qayyara on October 22. Hide Caption 64 of 73 Photos: Mosul: Iraqi-led forces push into key city Spent bullet cartridges litter the street around the Jihad Hotel, where ISIS militants battled Iraqi security forces in Kirkuk on Friday, October 21. Hide Caption 65 of 73 Photos: Mosul: Iraqi-led forces push into key city Gen. Abdel Ghani al-Asadi, who leads Iraq's counterterrorism forces, sits in Bartella on October 21 after the town was reclaimed. Hide Caption 66 of 73 Photos: Mosul: Iraqi-led forces push into key city Peshmerga fighters look over a village during an assault near Bashiqa on Thursday, October 20. Hide Caption 67 of 73 Photos: Mosul: Iraqi-led forces push into key city Iraqi forces head toward the front lines near Qayyara on Tuesday, October 18. Hide Caption 68 of 73 Photos: Mosul: Iraqi-led forces push into key city A Peshmerga fighter peers up from an underground tunnel in the liberated town of Badana on October 18. ISIS fighters have built tunnels below residential streets to escape from airstrikes. Hide Caption 69 of 73 Photos: Mosul: Iraqi-led forces push into key city Kurdish security forces take up a position near ISIS-controlled villages on Monday, October 17. Hide Caption 70 of 73 Photos: Mosul: Iraqi-led forces push into key city Smoke rises from a suicide car bomb attack carried out by ISIS in the village of Bedene on October 17. Hide Caption 71 of 73 Photos: Mosul: Iraqi-led forces push into key city Members of the Iraqi coalition gather around a fire at Zardak mountain ahead of the offensive. Hide Caption 72 of 73 Photos: Mosul: Iraqi-led forces push into key city Peshmerga forces deploy in the dark near the village of Wardak early on October 17. Hide Caption 73 of 73

Clashes in Kirkuk, Laylan

Kurdish security forces were going house to house Saturday in Kirkuk following a major ISIS attack a day earlier. Kirkuk is 175 kilometers (109 miles) southeast of Mosul.

Kirkuk's police chief said 48 ISIS militants were killed during hours of clashes.

Security officials told CNN that at least 40 others were killed and 76 wounded in the attack, the majority of them Kurdish Peshmerga.

The ISIS attacks continued in the area Saturday, with an attempt to infiltrate the town of Laylan, 20 kilometers (12 miles) southeast of Kirkuk. Nine militants were killed, according to the mayor of Laylan, Mohammed Wais, and some security force members were injured.

Kurdish President Masoud Barzani described ISIS' attack on Kirkuk as "a failed attempt by terrorists to make up for the defeats they have suffered at the hands of the Peshmerga on the front line."

Previous attacks by ISIS militants on Kirkuk have been attempts either to capture the city from the Peshmerga or divert Kurdish troops from the fight in Mosul.

Kurdish officials accuse local Arabs of helping ISIS

In the wake of the Kirkuk attack, Kurdish officials accused local Arabs and some displaced in camps around the city of helping ISIS.

Kirkuk's police chief, Brig. Gen. Khatab Omar, said the militants had probably infiltrated the 600,000 internally displaced in and around the city.

Gen. Hallo Najat, another police official, told local media to expect further fighting because 30% of the Arabs there supported ISIS rather than the government or Kurdish authorities.

Najmaldin Karim, Kirkuk's governor, said: "We have prior knowledge that an operation like this could happen; we were preparing for it, but the timing was not known exactly."

He said a curfew would remain in force for another day and urged the internally displaced community in Kirkuk to help track down militants.

US defense chief briefed on Mosul operation

Carter, the US defense secretary, paid an unannounced visit Saturday to Baghdad, where he was briefed on the Mosul offensive and met with Iraqi Prime Minister Haider al-Abadi.

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Carter then addressed about 50 US service members at Baghdad International Airport, recalling the US naval officer killed this week in northern Iraq and the risks taken by all those serving.

The defense chief told the crowd he was encouraged by what he has seen so far in the fight to retake Mosul.

"So far it is proceeding according to our plan," Carter said. "We've got tough fighting ahead and the US will continue to play its part."

US forces in Iraq are providing air support for the Mosul operation as part of an international coalition. US special operations forces are also advising Iraqi and Kurdish units on the ground.

After meeting with Carter, Abadi repeated his view that Iraq does not need Turkey's assistance in the battle for Mosul, while acknowledging the importance of maintaining good relations with its northern neighbor. "If we (need) help, we will ask for it from Turkey or from other regional countries," he said.

UN 'gravely worried' over human shield use

The United Nations expressed concern Friday that ISIS has taken 550 families from villages around Mosul to use as human shields.

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

Zeid Ra'ad Al Hussein, the UN High Commissioner for Human Rights, said, "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."

Iraqis flee violence

The first influx of Iraqis -- 144 people -- arrived at a new camp, Zelikan, set up to shelter what is expected to become a flood of families displaced by the Mosul offensive, the UN refugee agency said Saturday.

UN data indicate some 3,900 people -- or 650 families -- have so far been forced from Mosul and Hamdaniya districts, agency spokesman Adrian Edwards told a news briefing Friday in Geneva, Switzerland.

The UN refugee agency is working to establish 11 camps , five of which are already in place, to house those forced from their homes by the battle. The camps will have capacity for about 120,000 people, the agency said.

As many as 600,000 could be helped if the refugee agency obtained sufficient funding, it said. Mosul is believed currently to have a population of about 1.5 million people, it added.

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The charity Oxfam warned Saturday that more must be done to provide safe routes for those fleeing the conflict.

People who escaped from Hawd, 50 kilometers (about 30 miles) south of Mosul, told Oxfam that many civilians had been injured.

A woman told Oxfam her children had respiratory issues after breathing in thick smoke from oil wells that ISIS militants set afire to provide cover from coalition air attacks.

The smoke also concerned US and coalition troops at an airfield near Qayyara, about 60 kilometers (37 miles) south of Mosul. Forces were assessing Saturday whether the smoke was putting them at risk, and troops have been told to limit their outdoor activity, the US military said.