QAYARA, Iraq (AP) — At a checkpoint south of Mosul, men who once suffered under the harsh rule of the Islamic State group are now screening displaced civilians to identify any fleeing fighters, and some are dispensing what they call justice.

Dhia Amir said he has arrested more than 100 people — some as young as 13 — and handed them over to local police.

"Except for the men who I saw kill with my own eyes," Amir said earlier this week. "Those, I don't hand them over to the state. I deal with them myself."

Then he pointed to a sand berm on the side of the road where he said he shot two men. "I killed them right over there," Amir said.

Since the offensive to retake Iraq's second-largest city began Oct. 17, the Shiite-led government has tried to prevent revenge attacks against the mainly Sunni residents of Mosul and surrounding areas. State-sanctioned Shiite militias and Kurdish forces say they won't enter the city, and the government has vowed to investigate any human rights violations and hold people accountable.

But in chaotic areas near the front lines, the military is increasingly stretched thin and relying on local militias and tribal fighters to hold territory won back from the militants.

People who suffered from the group's vicious attacks and its bloody rule — marked by public beheadings, sexual enslavement and the use of child soldiers — say they are justly punishing known militants who deserve it.

Amir, who mans a checkpoint with soldiers, the militarized Federal Police and local militiamen, all in mismatched uniforms, said he is able to tell civilians from militants through a network of contacts and a government database.

"I have my sources," he said. "I'm from this area. I know a lot of people here."

He said he knew the two men he killed were IS militants because he saw them commit crimes. The Associated Press could not immediately confirm his account of the killings.

Amir said the extremist group, which swept across Iraq in 2014, capturing Mosul and massacring hundreds of captured soldiers and other opponents, can only be defeated through "harsh" tactics.

"A 13-year-old boy can still kill. And even if he doesn't kill me, one day he could kill my son," he said.

The U.N. mission to Iraq, Amnesty International and Human Rights Watch say they have not documented any abuses since the Mosul operation began. But both international rights groups found cases of extrajudicial killings, torture and illegal detention during previous assaults on IS-run towns and cities.

And while the forces leading the assault are widely seen as professional and nonsectarian, those who will have to keep the peace include Sunni tribes who are viciously split over allegiance to IS and other local fighters with bitter grievances.

Belkis Wille, senior Iraq researcher for Human Rights Watch, said if Iraq wants a lasting peace, "it can't maintain these campaigns of arbitrary detentions, forced disappearances and ill-treatment of prisoners."

"That is one of the reasons why we have IS today," she added.

The U.N. says it is closely monitoring the Mosul operation and the treatment of internally displaced civilians, or IDPs.

"We are monitoring through implementing partners, directly with people who have been detained and released, family members of people detained who have been sent on to IDP centers, and other reliable sources," said Francesco Motta, head of the U.N. mission's human rights office. He said they had one "unverified" report of a civilian being beaten while in custody, which they referred to authorities for investigation.

"We have not registered any violation of any kind by the security forces against civilians," said Brig. Gen. Yahya Rasool, a military spokesman. "Instead, the civilians are cooperating with and welcoming the security forces everywhere."

A group of Iraqi soldiers at Amir's checkpoint said they had heard reports of suspected IS fighters being beaten and killed by security forces. Pvt. Ali Hazoom said the alleged abuses were said to have been carried out by local tribal and militia fighters.

"Some of them, their father or their mother was killed by Daesh, so when they see the prisoners, they can't control themselves," he said, using an Arabic acronym for the group. "Of course, these practices will only make the situation worse," he added, "because revenge will only lead to more revenge."

Amir said he seeks justice, not revenge, and that he knows the difference.

"Some of the men, they plead with me, saying, 'I'm innocent, I'm innocent.' But if we find out they are lying, we beat them right there and they don't have another opportunity to speak," he said. "It's a great responsibility I have on my shoulders."

At a nearby camp for displaced civilians, a woman who gave only her nickname of Umm Laith out of fear of retribution broke down in tears as she described her family's flight from Shura, a village south of Mosul that saw heavy fighting.

She said their car was stopped Tuesday by uniformed men who did not identify themselves. They pulled her 16-year-old son, Qais, out of the car and took him away, saying she would never see him again.

"They beat him in front of me when they took him, but I couldn't do anything," she said. "I was afraid if I did anything they would take my other children."

Umm Laith acknowledged he had joined IS two years ago, when the militants took over their town, but added that he did it as a formality, to protect the family. "He wasn't a believer," she said.

Umm Laith recalls the terror in her son's eyes as he was being handcuffed and loaded onto the back of a pickup.

"His face was almost yellow he was so scared," she said. "I came here to protect my children, but they just took one of them away from me."

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Associated Press writers Joseph Krauss and Sinan Salaheddin in Baghdad and Salar Salim in Qayara, Iraq, contributed.

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Follow Susannah George on Twitter at www.twitter.com/sgreports . Her work can be found at http://bigstory.ap.org/journalist/susannah-george .