"The situation is so bad many people say it is worse than it has been in the times of Saddam Hussein," said Manfred Nowak, a UN special investigator on torture, at a press conference in Geneva.

He said government forces, private militia and terrorist groups were all involved.

"You have terrorist groups, you have the military, you have police, you have these militias. There are so many people who are actually abducted, seriously tortured and finally killed," said Mr Nowak, an Austrian law professor.

"It's not just torture by the government. There are much more brutal methods of torture you'll find by private militias."

Mr Nowak also said that bodies were being discovered with very heavy and very serious torture marks.

He said a mission to Iraq to investigate torture was too dangerous, but he had gathered information from interviews with people in Amman, Jordan, and other sources.

Mr Nowak is in Geneva to brief the UN Human Rights council - a body that addresses human rights violations - on the situation of the United States detention facility at Guantanamo Bay.

He is one of five UN human rights investigators who in February called for the closure of the camp on the grounds it was a "torture camp". The calls were rejected by the US.

Mr Nowak's comments come a day after the human rights office of the UN assistance mission for Iraq (Unami) raised concerns about the violence gripping the country. It said that 6,599 civilians had died in July and August.

Unami cited increasing evidence of violent torture, a growth in the numbers of death squads, and a rise in the honour killings of women and girls.

"Corpses appear regularly in and around Baghdad and other areas. Most bear signs of torture and appear to be victims of extrajudicial executions," said the report.