Iraqi security forces vehicles drive through Shuhadaa neighborhood in Falluja, Iraq, in this picture taken June 5, 2016. REUTERS/Stringer

GENEVA (Reuters) - The United Nations has “extremely distressing, credible reports” of Iraqi men and boys being abused by armed groups working with Iraqi security forces after escaping from the Islamic State-held town of Falluja, a senior U.N. official said on Tuesday.

The Iraqi army is assaulting Falluja, a bastion of Islamic State militants close to Baghdad, while U.S.- and Russian-backed forces are conducting separate offensives against the Sunni Muslim jihadists in neighboring Syria.

U.N. human rights chief Zeid Ra’ad al Hussein said in a statement that were also allegations of executions of men and boys who had fled Falluja into surrounding territory controlled by government forces and their Shi’ite Muslim militia allies.

“Eyewitnesses have described how armed groups operating in support of the Iraqi security forces are... detaining the males for ‘security screening’,” Zeid said.

“(This) in some cases degenerates into physical violations and other forms of abuse, apparently in order to elicit forced confessions.”

People escaping the town had suffered “two and a half years of living hell” under Islamic State rule and faced not just enormous danger in escaping but also “double jeopardy in the form of serious human rights violations”, Zeid said.

Iraqi authorities have a legitimate interest in vetting anyone fleeing Islamic State-held areas to be sure they were not a security risk or suspected of involvement in atrocities, but the process had to be carried out by legal organs, Zeid said.

“It is paramount that all individuals fleeing the violence around Falluja must be assumed to be civilians without links to armed groups, unless there is clear and cogent evidence to the contrary,” he said.

Zeid called on Iraq’s Shi’ite-led government to immediately take concrete steps to ensure it was putting into practice its pledges to protect civilians - Falluja’s population is mainly Sunni - and bring people to justice for violating human rights.