John Pace, who left Baghdad two weeks ago, said up to three-quarters of the corpses stacked in the city's mortuary show evidence of gunshot wounds to the head or injuries caused by drill-bits or burning cigarettes. Much of the killing, he said, was carried out by Shia Muslim groups under the control of the Ministry of the Interior.

Much of the statistical information provided to Pace and his team comes from the Baghdad Medico-Legal Institute, located next to the city's mortuary. He said figures show that last July alone the morgue received 1,100 bodies, about 900 of which bore evidence of torture or summary execution. The pattern prevailed throughout the year until December, when the number dropped to 780 bodies, about 400 of which bore gunshot or torture wounds.

"It's being done by anyone who wishes to wipe out anybody else for various reasons, " said Pace, who worked for the UN for over 40 years in countries ranging from Liberia to Chile. "But the bulk are attributed to the agents of the Ministry of the Interior."

Coupled with the suicide bombings and attacks on Shia holy places carried out by Sunnis, some of whom are followers of Abu Musab alZarqawi, al-Qaeda's leader in Iraq, the activities of the death squads are pushing Iraq ever closer to a sectarian civil war.

Pace said the interior ministry was "acting as a rogue element within the government". It is controlled by the main Shia party, the Supreme Council for Islamic Revolution in Iraq (Sciri); the interior minister, Bayan Jabr, is a former leader of Sciri's Badr Brigade militia, one of the main groups accused of carrying out sectarian killings.

Another is the Mehdi Army of the young cleric, Moqtada alSadr, who is part of the Shia coalition seeking to form a government after winning the mid-December election.

Many of the 110,000 policemen and police commandoes under the ministry's control are suspected of being exmembers of the Badr Brigade.

Apart from counter-insurgency units such as the Wolf Brigade, the Scorpions and the Tigers, the commandoes and even the highway patrol police have been accused of acting as death squads.

The paramilitary commandoes, dressed in garish camouflage uniforms and driving around in pick-up trucks, are dreaded in Sunni neighbourhoods. People whom they have openly arrested have frequently been found dead several days later, with their bodies bearing obvious marks of torture.

Pace, a Maltese-Australian who has now retired from his UN post to his home in Sydney, says the constant violence and utter lack of security in Iraq is creating a vicious circle in which ordinary citizens are turning to extremist sectarian groups for protection. Fear of anybody in official uniform inevitably strengthens the militias and the insurgents.

In Sunni areas, people will look to their own defences and not to the regular army and police.

But ordinary Sunnis are caught between the death squads and the desire of some of the insurgents on their own side to start a civil war , , an aim they are now not far off achieving. The so-called Salafi, Sunni fundamentalists, want not only to eject the Americans but to build a pure Islamic state. It is they who see Iraqi Shia, though they comprise 60% of the population, as heretics allied to the US who should be slaughtered.

Last week's attack on the Golden Mosque is just the latest in a long series of outrages against the Shia community, starting in August 2003 when Mohammed Baqr al-Hakim, then leader of Sciri, was killed along with over 100 of his followers by a suicide bomber in a vehicle outside the Imam Ali shrine in Najaf. There have been repeated massacres of the Shia ever since; some have targeted the security forces, such as the attacks on queues of young men trying to join the police or army, but others, such as the slaughter of Shia day labourers waiting for a day's employment, for no other reason than that they are Shia.

Despite extending a 24hour curfew yesterday in Baghdad and other major cities, the authorities were unable to prevent further revenge killings and outrages against holy sites. The current cycle of violence, which began with the bombing of the Azkariya shrine in Samarra on Wednesday, has claimed at least 200 lives so far, including those of 47 factory workers pulled from buses and shot on the outskirts of Baghdad.

This was the sort of killing that touched off the Lebanese civil war in 1975. Already an exchange of populations is taking place in Baghdad as members of each community move to districts in which they are the majority.

The ability of the US occupiers to influence the situation is not only limited, but some of their actions are seen as making things worse. The Americans have been trying to dislodge Jabr as interior minister, accusing him of turning his ministry into a Shia bastion. But the Shia believe the US and its allies, the Kurds, simply want to prevent the majority community from gaining full power over security, despite winning two parliamentary elections in 2005.

One important development over the last few days is that it is clearly becoming very difficult to use American or British troops to keep the peace, undermining the argument that they are the only bulwark against civil war. The occupation forces lack the legitimacy to play the role of UN peacekeepers; it is almost impossible to have US soldiers defend a Sunni mosque against a Shia crowd, because if they open fire they will be seen as having joined one side in a sectarian struggle.

In Pace's view, the violence in Iraq is being made worse by the seizing of young Iraqi men by US troops and Iraqi police as they move from city to city carrying out raids.

"The vast majority are innocent, " he said, "but they very often don't get released for months. You don't eliminate terrorism by what they're doing now. Military intervention causes serious human rights and humanitarian problems to large numbers of innocent civiliansf The result is that such individuals turn into terrorists at the end of their detention."

In such circumstances, family members often contacted UN officials asking for help in getting a young man out of the country and away from the influence of insurgents he had met in jail. They were among many Iraqi citizens fleeing the country as a result of the violence.

"Those with money go to Jordan, the poor go to Syria, " he said.

Pace, who first made his comments to the Times of Malta newspaper, said the situation in Iraq had "definitely, definitely" got worse in the two years he headed the UN human rights team. The interim government and the international community were trying to restart the country's crippled economy, but they would not succeed "until people are secure".