Iraqi Prime Minister Haider al-Abadi on Tuesday vowed that those responsible for abuses against civilians would be punished as allegations of torture and killing marred the third week of a campaign to recapture Fallujah from Islamic State militants.
The local provincial council meanwhile backed claims by refugees from the city that pro-government Shiite militias were responsible for the alleged abuses.
The council called on al-Abadi to pull the militias, known as the Popular Mobilization, out of operations as security forces close in on the Sunni-populated city west of Baghdad.
The militias, theoretically under government control, are politically powerful and have provided crucial backing to security forces.
But the allegations highlight a potential threat to the long term success of operations against Islamic State, as alleged sectarian abuses against Sunnis have previously been a powerful recruiting tool for the jihadists.
Refugees from Fallujah and nearby areas who spoke to dpa said that the militias had subjected them to beatings and mistreatment during screening aimed at ensuring that jihadists were not escaping along with the civilians.
Members of one family said that 17 of their male relatives, including at least one 10-year-old boy, had been summarily executed by militiamen wearing police uniforms.
UN human rights commissioner Zeid Ra'ad Al Hussein called on authorities to fully respect the rights of displaced people undergoing security screening.
"There are extremely distressing, credible reports that some people who survive the terrifying experience of escaping from ISIL, then face severe physical abuse once they reach the other side," Al Hussein said.
Al-Abadi said that those accused of abuses had been arrested and referred to the judiciary "to receive their punishment in accordance with the law."
But he also warned against "believing or exaggerating rumours," and praised "the humanitarian treatment of the refugees by the Iraqi forces."
US State Department spokesman Mark Toner called the reports "obviously concerning," while adding that "any kind of screening, while justified, needs to be done in a manner that is respectful of human rights and common dignity and also done in a transparent manner."
Elite counter-terrorism forces have led operations against the jihadists who control the Sunni-populated city, but the militias have been involved in clearing operations in outlying districts.
Ahmed Saad, a civilian who had escaped an area near Fallujah, told dpa that security forces brought him with other refugees to a camp where they were questioned and "subjected to beatings, insults and curses from members of the Popular Mobilization.”
Four civilians had died during questioning because they were "savagely beaten" with truncheons, but appeals to security commanders brought no results, said Hussein Awad, another refugee.
A 51-year-old tribesman from al-Sejar, outside Fallujah, told dpa that 17 young men and boys from his extended family had been summarily executed by a militiaman wearing police uniform.
Islamic State members had forced the family group to move from al-Sejar to another area where they slept in a house near a school, the tribesman, who identified himself as Abu Mohammed, said.
When they saw approaching security forces, they raised a white flag and thought they had been liberated, he said.
But the forces, who were wearing police uniforms but carrying the flag of a militia group, separated the men from the women and then took the 17 young men and boys aside and shot them dead, he said.
Abu Mohammed's nephew, Abu Saad, added that police subsequently protected the remainder of the group and took them to a militia headquarters where they were questioned for two days and then released.
dpa was not able to independently corroborate Abu Mohammed's account, although it appears to correspond to killings reported by local lawmaker Hamid al-Mutlaq last week.
Also on Tuesday, the military denied claims by officials two days earlier that a mass grave containing the bodies of 400 troops had been found in a district near Fallujah.