As U.S.-backed Iraqi forces open their offensive for the northern city of Mosul, an international rights group warned Tuesday that civilians fleeing Islamic State-held territory face potential violence from government troops and allied militias who it says have tortured, detained and killed people escaping militant-controlled areas in the past.

In a new 70-page report, Amnesty International documents what it describes as war crimes committed by Iraqi forces and militiamen against civilians uprooted by previous military operations. The rights group raises the possibility of similar abuses on an even greater scale during the operation to capture the Islamic State stronghold of Mosul, a majority Sunni city where an estimated 1 million civilians are believed to be trapped.

“After escaping the horrors of war and tyranny of IS, Sunni Arabs in Iraq are facing brutal revenge attacks at the hands of militias and government forces, and are being punished for crimes committed by the group,” said Philip Luther, Amnesty’s research and advocacy director for the Middle East and North Africa, in reference to the Islamic State. “As the battle to retake Mosul gets underway, it is crucial that the Iraqi authorities take steps to ensure these appalling abuses do not happen again.”

The Islamic State overran Mosul in July 2014 as the militants seized control of much of northern and western Iraq. On Monday, thousands of Iraqi forces, including government troops, Shiite militias and Kurdish fighters known as Peshmerga launched an offensive to take back the city from the extremists.

Wary of how Shiite militias could be perceived by Sunni civilians, officials say the militiamen are not authorized to enter Mosul itself, although they are expected to have some role in the operation.

[U.S. Military Plans Tough Battle to Regain Mosul]

In its report, Amnesty documents the abuses that Shiite militias, as well as some government troops, have been accused of committing against Sunni civilians who lived under Islamic State rule.

During the military operation earlier this year to retake the city of Fallujah from the Islamic State, Amnesty says Shiite militias detained, tortured and killed men and boys from the nearby town of Saqlawiyah.

It says a large Shiite militia force took some 1,300 men and older boys away on June 3, and three days later transferred 605 of them, many of whom bore marks of torture, to local officials.

One survivor told Amnesty that the men were beaten with metal rods, shovels, pipes and cables. An investigative report published on June 11 by the governor of Anbar province said the militiamen killed 49 of the original detainees, while 643 were still missing.

The rights group warns that such cases are not isolated incidents. It says that thousands of Sunni men and boys have gone missing after being taken into custody by Iraqi security forces and militias since the rise of the Islamic State and other Sunni extremist groups.

Amnesty urges the United States and European countries aiding Iraq in the fight against the Islamic State to make their assistance contingent upon Iraqi authorities taking concrete steps to prevent future abuse.

The Amnesty report is based on interviews with more than 470 former detainees, witnesses and relatives of those killed or detained. The group also spoke with officials, activists and aid workers.