The U.N. human rights chief is flagging "extremely distressing, credible reports" that Iraqis fleeing Fallujah are facing physical abuse and even cases of summary executions as they escape the city held by Islamic State militants.

Zeid Ra'ad al-Hussein on Tuesday cited witness accounts that armed groups backing Iraqi security forces have detained some men and teenage boys leaving Fallujah with force that at times "degenerates" into abuse. Zeid acknowledged that Iraqi forces have a "legitimate interest in vetting individuals fleeing ISIL-controlled areas" to make sure they don't pose a security risk, but said official authorities should do so.

He said those fleeing must be presumed to be civilians, barring "clear and cogent evidence to the contrary." He urged the Iraqi government to take steps to ensure the protection of civilians.