IRBIL, Iraq (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 mainly 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 of 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 a handful of neighborhoods in the city — the country's second largest — since the operation began Oct. 17.

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.

Associated Press writer Sinan Salaheddin in Baghdad contributed to this report.