BAGHDAD (AP) — A militia backed by the Iraqi government killed suspected Islamic State fighters captured during the operation to retake Mosul, Human Rights Watch said Sunday.

The Hashed al-Jabour militia, made up of Sunni tribal fighters, killed four men it had captured in a village north of Mosul in November, according to a report published by the New York-based group. The report cited witnesses who said the men were shot in the presence of Iraqi security forces without any judicial proceedings.

The militia is part of the Popular Mobilization Forces, a group of mostly Shiite militias sanctioned by the government which have been accused of abuses during past campaigns against IS, a Sunni extremist group.

Iraqi government spokesman Saad al-Hadithi said authorities were unaware of the incident, but were committed to arresting and trying anyone suspected of human rights violations.

"Generally speaking, retaliations could happen in some areas by the locals (in the PMF) who had family members and relatives killed by Daesh before the entering of government security forces," al-Hadithi said, using the Arabic acronym for IS. "Such acts are totally rejected by the Iraqi government and are fully investigated, and those behind it face trials."

In comments broadcast on state television Saturday, Iraq's Prime Minister Haider al-Abadi said he had not received any "complaints" about the Popular Mobilization Forces. He said the Mosul fight was "clean," and moving forward at a "good pace."

The Mosul offensive involves tens of thousands of Iraqi soldiers and police, as well as Kurdish fighters, Shiite militias and Sunni tribesmen. Iraqi forces have seized around a quarter of the city — the country's second largest — since the operation began Oct. 17.

"This was a very difficult fight," Lt. Gen. Abdul-Ghani al-Asadi of the special forces said in Mosul's al-Barid neighborhood on Sunday. As Iraqi troops have pushed into denser neighborhoods, their advance has been slowed by heavy resistance.

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The International Organization for Migration said Sunday that at least 103,872 people have fled their homes since the operation began. The city was still home to more than a million people when the offensive began two months ago.

Human Rights Watch has previous accused Sunni militias participating in the Mosul operation of recruiting boys younger than 18 for the fight. The group has also accused the Shiite militias operating under the PMF of abuses against civilians in majority Sunni towns and cities. The prime minister's office has investigated individual militiamen after past allegations.

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Associated Press writer Susannah George in Mosul, Iraq contributed to this report.