Iraq is descending into savage sectarian warfare as government-backed Shia militias kill, torture and hold for ransom any Sunni whom they detain. Isis is notorious for its mass killings of Shia, but retaliation by Shia militiamen means that Iraq is returning to the levels of sectarian slaughter last seen in the Sunni-Shia civil war of 2006-07 when tens of thousands were murdered.

The Shia militias have become the main fighting force of the Baghdad government since the Iraqi army was defeated by Isis when it took northern Iraq in June. According to a detailed Amnesty International report published today, the militias enjoy total immunity in committing war crimes against the Sunni community, often demanding large ransoms but killing their victims even when the money is paid.

The re-emergence of the Shia militias and the failure to rebuild the Iraqi army is torpedoing the US and British policy of supporting a more inclusive and less sectarian government in Baghdad. The aim was to create a government that could reach out to Iraq’s five or six million-strong Sunni community and seek to turn it against Isis. But, since the militias treat all Sunni men as Isis fighters or supporters, the Sunni are left with no choice but to stick with the jihadi militants.

The report cites a member of the Asa’ib Ahl al-Haq, one of the largest militias, on duty at a checkpoint north of Baghdad, saying: “If we catch ‘those dogs’ [Sunni] coming down from the Tikrit we execute them; in those areas they are all working with Daesh [Isis]. They come to Baghdad to commit terrorist crimes. So we have to stop them.”

The aftermath of a car-bomb explosion in the mostly Shia Sadr City district of Baghdad on Tuesday, one of three within an hour (Getty) (Getty Images)

In addition to sectarian motives, militias such as Asa’ib Ahl al-Haq, the Badr Brigades, Kata’ib Hezbollah and Saraya al-Salam are thoroughly criminalised. One mother said: “I begged friends and acquaintances to lend me the ransom money to save my son, but after I paid they killed him and now I have no way to pay back the money I borrowed, as my son was the only one working in the family.”

Moving on the roads has become lethally dangerous for Sunni even before Isis launched its summer offensive. On the afternoon of 30 May two cousins, Majed, a 31-year-old ministry of education employee and father of three, and Nayef, an engineer, were abducted at a checkpoint when they went to Tikrit from Baghdad to pick up furniture.

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A $90,000 (£56,000) ransom was demanded and paid but they were later found handcuffed and shot in the head. A Sunni businessman called Salem, 43, was kidnapped from his factory at al-Taji and, though a ransom was paid, his body was later found with his head smashed in either by a large calibre bullet or some form of club.

American and British ministers have lauded the new government in Baghdad under Prime Minister Haider al-Abadi as being less sectarian than that of his predecessor, Nouri al-Maliki, whom he replaced in August.

But in practice, Mr Abadi’s administration is much like the old. “For now nothing is different,” says Donatella Rovera, Amnesty International’s senior crisis response adviser. “Shia militias are way more important than the army and are running the show.” Even if it wanted to the government would have difficulty in bringing them under control. Ms Rovera says: “In terms of sectarian violence we are back to the levels of 2006-07.”

The militia gunmen frequently act in co-ordination with the police and army. Their victims are Christian as well as Sunni. One Christian family, threatened with death by three militiamen unless they paid a large sum, fled the country but without telling the police.

The report comments that the fact that the family thought it would be unsafe and unwise to tell the police “speaks volumes about the atmosphere of lawlessness in the capital, where [Shia] militias know they can act with impunity.”

One reason the Sunni community first protested and supported armed resistance against the government has been the knowledge that they could be detained and tortured by government forces at any time.

Uda Taha Kurdi, 33, a lawyer, was arrested at the Baghdad Central Court on 10 June. Two weeks later his family was told he had suffered from “a health problem” and had died, a judge alleging that he was “from a terrorist family” and was “from the IS leadership”. A forensic examination of Mr Kurdi’s body concluded that he had probably been killed by electrical torture with electrodes attached to his calf and little toe.

Shia militia fighters brandish their weapons in the shrine city of Karbala before joining the fight against Isis (Getty) (Getty Images)

The overall plan of Mr Obama and his allies to find a reliable ally on the ground in Baghdad who could woo the Sunni has failed to make progress, despite the departure of Mr Maliki. Mr Abadi has still to get his choice for the defence and interior ministries accepted by parliament.

Meanwhile, Isis has seized all of Anbar province west of Baghdad, defeating the Iraqi army despite the support of US airpower. One of the last two army bases in Anbar fell on Monday as Isis began moving towards west Baghdad.

The inability of the Baghdad government to field a national army and its reliance on militias means that Iraq is in the last stages of disintegration. The few mixed Sunni-Shia areas are disappearing.