The trek to Baghdad's mortuary is a familiar one for many people

John Pace, until recently UN human rights chief in Iraq, told the BBC News website that up to 75% of the corpses showed signs of extrajudicial death.

Mr Pace blamed an "endemic" breakdown of security for increasing violence.

"Anyone with a gun who is reasonably well organised can do whatever they want with impunity," he said.

Armed groups often threatened mortuary staff, aiming to stop autopsies and suppress evidence, Mr Pace said.

Iraq has seen a jump in apparently sectarian violence since the bombing last week of a Shia Muslim shrine in the city of Samarra.

But Mr Pace played down suggestions that Iraq was heading towards civil war, blaming a political vacuum and the collapse of law and order - rather than a generalised Shia-Sunni split - for the escalating violence.

'Chaotic'

Mr Pace described the Baghdad mortuary as a "barometer" of the situation in the city at any one time.

Staff at Baghdad's mortuary face regular threats

Between 780 and 1,110 corpses had been brought in every month over the past year, Mr Pace said.

Two-thirds to three-quarters bore signs of death by deliberate gunshot or signs of torture before death, he added, describing daily life as "chaotic".

He confirmed that the head of Baghdad's mortuary had left his position temporarily amid fears for his security.

Despite threats, most staff continued to perform autopsies, Mr Pace said.

Failures

Mr Pace's comments came as Sunni officials said that dozens of Sunni preachers had been killed in a wave of violence since the bombing of Samarra's al-Askari shrine.

Elsewhere, talks on the formation of a new national government were delayed.

Fears have grown among Iraqis that Shia death squads are operating with semi-official government approval.

While highly critical of Shia militias such as the Badr brigade, linked to Interior Minister Bayan Jabr, Mr Pace said that Iraq's real problems stemmed from a lack of political authority, a breakdown of law and order and a weak judicial system.

"There is no deliberate intention to suppress information [about executions and killings]," he said.

"More likely it is considered as something [the government] would rather not admit to."