BAGHDAD (Reuters) - The United Nations said on Wednesday it had received further reports alleging “serious rights violations” of civilians fleeing the Iraqi city of Falluja by armed groups supporting a U.S-backed military offensive against Islamic State.

Destroyed buildings from clashes are seen on the outskirt of Falluja, Iraq, June 20, 2016. REUTERS/Thaier Al-Sudani

One month into their assault, Iraqi forces are battling to retake remaining districts in the city’s north and west.

Thousands of civilians have fled the predominantly Sunni Muslim city. Last week, the provincial governor said 49 Sunni men were executed after surrendering to a Shi’ite militia supporting the military, and more than 600 are missing.

A spokeswoman said the United Nations High Commissioner for Human Rights had obtained video footage of two further incidents purporting to show abuses of civilians from Falluja. In one, a man was beaten around the head and dragged along the ground by a military truck, and another showed people being struck with a rifle and kicked in the head.

In both cases the perpetrators were wearing military uniforms, but the U.N. said it could not identify their affiliation, nor could it authenticate the videos. However, it had received reports of such violations from several sources, the spokeswoman said.

The U.N. has previously called on the Iraqi government to ensure accountability over the abuse allegations, which threaten to unleash further sectarian violence in Iraq where it has already cost thousands of lives. Several military personnel have been arrested.

A spokesman for the Hashid Shaabi, a government affiliated coalition of mostly Iranian-backed Shi’ite Muslim militias, said the reports were likely false and part of a smear campaign.

“Unfortunately the Iraqi security forces and the Hashid Shaabi are still under a large systematic media attack that aims to disfigure the victory in Falluja,” said Youssef al-Kilabi. “There are many reports, video clips and news by some channels which are either old or faked.”

Kilabi said Islamic State militants had tried to smuggle their families out alongside the estimated 85,000 civilians who have fled Falluja since the offensive began, heading for government-run camps.

Falluja, a bastion of Sunni insurgency against the U.S. forces that toppled Saddam Hussein, a Sunni, in 2003, was seen as a launchpad by Islamic State for suicide bombings in Baghdad, an hour’s drive away, which often target government forces and Shi’ite civilians.