The UN's top human rights official has called for an international probe into the conditions for detainees in Iraq.
Sunni leaders say torture is endemic in government jails
Louise Arbour said it was "essential" following allegations of torture at an interior ministry detention centre.
"An international element would help address problems in the system in an impartial and objective way," she said.
The authorities started their own inquiry after 170 prisoners, some showing signs of apparent torture, were found by US troops on Sunday.
Iraqi officials promised a full investigation and said the perpetrators would be punished, but Sunni Arab groups said only an international inquiry would have credibility.
Most of the detainees are believed to have been Sunni Arabs - the main group involved in the insurgency.
Sunni leader Saleh Mutlaq has accused the government of holding more than 1,100 prisoners at the ministry and suggested a number of them had been tortured to death.
"In the light of the apparently systemic nature and magnitude of the problem, and the importance of public confidence in any inquiry, I urge the authorities to consider calling for an international inquiry," Ms Arbour said in Geneva.
Ms Arbour - who is UN High Commissioner for Human Rights - also expressed concern about the high number of people held in detention in Iraq.
"There is an urgent need to provide remedy to lengthy internment for security reasons without adequate judicial oversight," she said.
Iraqi Interior Minister Bayan Jabr said reports of prisoners being tortured at the Jadiriya centre in Baghdad were "exaggerated".
He said that at most seven detainees had shown signs of beating and he rejected accusations that he had condoned torture by a Shia militia linked to his ministry.