Acting High Commissioner for Human Rights Bertrand Ramcharan also suggested in a new report that US soldiers accused of human-rights violations in Baghdad's Abu Ghraib prison could be guilty of war crimes.
There have been "serious human-rights problems" under the Coalition Provisional Authority including the jailing of large numbers of people "without anyone knowing how many, for what reasons ... and how they were being treated", he said.
Ramacharan's report, for the UN's Human Rights Commission, quoted interviewed Iraqis who spoke of "arbitrary arrests and detention as an ongoing phenomenon" since US-led forces occupied Iraq in March 2003.
In a clear reference to Abu Ghraib, Ramcharan said "wilful killing, torture or inhuman treatment" of detainees are a grave breach of international law.
Such acts, he added, "might be designated as war crimes by a competent tribunal".
Ramcharan, a British-trained barrister from Guyana and long-time UN official, said that the occupation authorities "should appoint immediately an International Ombudsman or Commissioner" on human rights.
Too high price
The report, which was commented on by US and British authorities prior to its release, acknowledged the removal of Saddam Hussein's government as "a major contribution to human rights in Iraq".
Released Iraqi prisoners said
they were tortured in Abu Ghraib
But it cited various reports of mistreatment by troops of Iraqi men, women and children, and declared that the foreign forces in the country had in effect "immunity" from any impartial jurisdiction for wrongful acts and rights abuses.
In this regard, Abd al-Bari Atwan, editor-in-chief of the London-based Arabic newspaper al-Quds al-Arabi, told Aljazeera.net the US contribution to human rights in Iraq may have come at too high a price.
“Fourteen months since the occupation, the people do not have full access to basic needs like pure water and electricity, [must endure] miserable medical care, and above all, are faced with the lack of security," he said.