"The situation as far as torture is concerned in Iraq is now completely out of hand," Manfred Nowak, the UN special rapporteur on torture and cruelty, said in Geneva.
"The situation is so bad that many people say that it is worse than in the times of Saddam Hussein," he said.
Mr Nowak, who has not visited Iraq, said he had credible reports of torture in facilities run by Iraqi forces, as well as by the militias and insurgents who have kidnapped and killed hundreds of people since Saddam's overthrow in 2003.
"The situation as such is extremely serious, but it is not just torture by the government," Mr Nowak said.
The most brutal torture methods were employed by private militias, Mr Nowak told journalists.
The report also says the frequency of sectarian bloodletting means bodies are often found which "bear signs indicating that the victims have been brutally tortured before their extra-judicial execution".
It concludes that torture threatens "the very fabric of the country" as victims exact their own revenge and fuel further violence.
He also cited reports of inhumane treatment in US- and foreign-run detention centres, but said conditions there seemed to have improved since an international furore over mistreatment of prisoners by US forces at the Abu Ghraib prison.
Abu Ghraib was handed over to Iraqi control three weeks ago.
UN urges investigation
The UN Assistance Mission in Iraq has urged the Iraqi government to invite Mr Nowak to go to Iraq to investigate allegations of torture in prisons run by the US-led multinational force as well as by the Interior and Defence ministries and by militias.
Bodies found in the Baghdad morgue "often bear signs of severe torture including acid-induced injuries and burns caused by chemical substances, missing skin, broken bones - back, hands and legs - missing eyes, missing teeth and wounds caused by power drills or nails," the mission said.
Mr Nowak, who was in Geneva to address the UN Human Rights Council, said he had held talks with Iraqi authorities in June about a fact-finding mission, but had not yet received an invitation.
Mr Nowak said he would like to visit Iraq in person, but the current situation would not allow him to prepare an accurate report, because it would not be safe to leave Baghdad's heavily guarded Green Zone where the Iraqi government and US leadership are situated.
Mr Nowak has a mission scheduled for October 9-20 to Russia, including visits to Chechnya and North Ossetia, and will travel to Sri Lanka at the end of January.
He has also received an invitation from Indonesia for a mission in 2007, following a request first made in 1993.