Iraqis who fled fighting around Fallujah gather at a camp run by the Norwegian Refugee Council. (Ahmad Mousa/AFP/Getty Images)

Islamic State militants have gunned down people trying to flee the Iraqi city of Fallujah even as pro-government forces have apparently committed beatings and torture during battles to retake the area, according to Human Rights Watch and Iraqi officials.

The accounts — which could not be independently verified — highlight the apparent desperation of the militants to hang onto the city, their closest main foothold to Baghdad.

But the accusations also point to Iraq’s deep sectarian and tribal rifts, which could complicate further fights to dislodge the Islamic State, including from its key base in northern Iraq.

Islamic State militants have used the estimated 50,000 civilians trapped in the government-besieged city as human shields, and they have killed those who have attempted to escape, the rights group and officials told The Washington Post in advance of a report on the alleged abuses.

In turn, they say, government-aligned Shiite militiamen have seized and severely beaten men fleeing Fallujah and neighboring areas. At least four of those men died from their injuries, according to local officials familiar with the incident.

The allegations underscore how a U.S.-supported offensive to drive Islamic State militants out of the war-torn country could be undercut by fraught relations among Iraq’s religious groups.

The involvement of government-aligned Shiite militias has raised concerns about sectarian friction in Fallujah, located in Iraq’s Sunni heartland about 45 miles west of Baghdad. Many Sunnis — even those battling the Islamic State — are uneasy about the power of Shiite militia groups that have gained prominence in Iraqi affairs since the 2003 fall of Saddam Hussein in a U.S.-led invasion.

“We’re receiving more and more reports of abuses, including violations by ISIS and possible summary executions by government forces against civilians that violate the laws of war,” said Belkis Wille, an Iraq-based researcher with Human Rights Watch. ISIS and ISIL are acronyms for the Islamic State. The report by Human Rights Watch is expected to be made public this week.

The findings are partly based on interviews with the local councilmen of Saqlawiyah, a suburb of Fallujah. The local leaders said they have formally complained to Iraqi Prime Minister Haider al-Abadi’s office about the alleged abuse of at least 1,250 men who are said to have been detained and beaten by pro-government Shiite militiamen.

The militias seized the men at the beginning of the assault on Fallujah and held them incommunicado at commandeered homes in Saqlawiyah, two councilmen said by telephone. They said that the detained men, mostly from the al-Aqash tribe, were brutally beaten with metal rods and cables, four of them fatally.

Iraqi authorities have barred human rights groups from interviewing the alleged victims, Wille said.

On Sunday, the Saqlawiyah councilmen said, the militias released 650 of the detained men at Camp Tariq, where government forces are overseeing the assault on Fallujah.

“They have been so badly beaten. Their faces are bludgeoned. They have fractured bones and deep lacerations on their bodies and faces,” said Abdullah Ismael Moukhlif, a Saqlawiyah councilman who said he interviewed some of the formerly detained men. Local police are interrogating a number of them for suspected ties to the Islamic State, but many are civilians, he said.

Moukhlif added that those who were released have claimed that some of the 600 other detained men — who are still missing — were summarily executed. The claim could not be independently verified.

The pan-Arab Al Jazeera television network aired footage Monday of the alleged victims, showing heavy bruising and lacerations on their bodies.

Saad al-Hadithi, a spokesman for the Iraqi prime minister, said a number of people have been arrested in the Fallujah area for unspecified violations. He said the government has begun an investigation.

“These actions are individual actions and do not represent government forces or the popular mobilization units,” Hadithi said, using the umbrella term for government-aligned militias.

Yousif al-Kalaby, a spokesman for the mainly Shiite militias, denied that they were involved in any such abuses.

Iraq’s government has told the Shiite militias to avoid entering Fallujah, where Sunni residents resent and fear the country’s Shiite-dominated government.

Sunni officials worry that militiamen want to exact revenge on Fallujah residents for Islamic State attacks, including a recent spate of bombings in Baghdad.

Since the start of the campaign to retake Fallujah, the extremist group apparently has been using lethal force to prevent civilians from fleeing the city, according to Human Rights Watch, aid workers and Iraqi officials.

“From anecdotal evidence collected from fleeing people, we’re seeing a pattern where civilians in Fallujah are being used as human shields” to stop the government’s advance, said an aid worker from a Western humanitarian organization who spoke on the condition of anonymity because of security concerns.

Some fleeing residents appear to have been fatally shot by Islamic State fighters, and others have been targeted with artillery fire as they tried to leave, the aid worker said.

Several people have drowned in the nearby Euphrates River as they tried to escape Fallujah in recent days, according to the International Rescue Committee. The aid group said the Islamic State apparently destroyed boats to make it harder for residents to cross the river.

Shiite militiamen in the area also accused the Islamic State of using suicide bombers in Saqlawiyah last week to attack fleeing civilians and deter others from leaving areas it controls.

Mustafa Salim contributed to this report.