Iraq's Shia militias have abducted and killed "scores" of Sunni civilians with the tacit support of the government in retaliation for ISIS group attacks, Amnesty International said Tuesday, as a suicide car bombing killed 23 people, including a Shia lawmaker.

The Shia militiamen number in the tens of thousands and wear military uniforms but operate outside any legal framework and without any official oversight, the London-based watchdog warned in its new report, entitled "Absolute Impunity: Militia Rule in Iraq." It said the militiamen are never prosecuted for their crimes.

The accusations were based on interviews with families and survivors who claimed that members of four prominent Iraqi Shia militias — Asaib Ahl al-Haq, the Badr Brigades, the Mahdi Army, and Ketaeb Hizbollah — were behind the abduction and killing of many Sunnis.

Mideast Iraq

Shiite militiamen hold a flag of the Islamic State group they captured during an operation north of Baghdad. Amnesty International says they operate outside any legal framework and without any official oversight. (Associated Press)

Sunni insurgents have regularly targeted Shia neighbourhoods with car bombs and other attacks since the 2003 U.S.-led invasion, and in June the ISIS extremist group swept across northern Iraq, seizing the second largest city Mosul.

In the aftermath of the onslaught, then-Prime Minister Nouri al-Maliki called on volunteers to support the Iraqi army, leading several powerful militias — many with links to neighbouring Iran — to mobilize to defend the country.

The revival of the militias has deepened the sense of alienation among the country's Sunni minority — seen as a key factor behind the rise of ISIS — and has raised fears of a return to the sectarian conflict that gripped the country in 2006 and 2007.

Granting its blessing

Iraq's new Prime Minister Haider al-Abadi, from al-Maliki's Shia Islamist Dawa party, has pledged to bring the militants under control, but Amnesty said the government has not only failed to prosecute Shia militiamen but has openly condoned their actions.

"By granting its blessing to militias who routinely commit such abhorrent abuses, the Iraqi government is sanctioning war crimes and fuelling a dangerous cycle of sectarian violence that is tearing the country apart," said Donatella Rovera, a senior adviser with Amnesty.

"Shia militias are ruthlessly targeting Sunni civilians on a sectarian basis under the guise of fighting terrorism, in an apparent bid to punish Sunnis for the rise of the (ISIS group) and for its heinous crimes," she added.

Amnesty said the fate of many Sunni abductees remains unknown and that some captives have been killed even after their families paid ransoms of $80,000 or more.

The Shia militias have done little to roll back ISIS's gains and insurgents have continued to target Shia neighbourhoods of the capital on a near-daily basis.

Wounded 52

On Tuesday, a suicide bomber rammed his explosives-laden car into a security checkpoint in a Shia-majority neighbourhood in northern Baghdad, killing at least 23 people, including Shia lawmaker Ahmed al-Khafaji, police said. The police said al-Khafaji was not the target in the attack, but just happened to be driving through the Kazimiyah neighbourhood when the attack took place.

The explosion also wounded 52 people. Dozens of cars were either burnt or damaged.

Hospital officials confirmed the casualties. All officials spoke on condition of anonymity because they are not authorized to speak to media.

On Monday, a series of attacks on Shia-majority neighbourhoods in Baghdad killed 44 people.

ISIS claimed responsibility for two of those attacks in a statement posted on extremist websites Tuesday. The statement could not immediately be verified but it was posted on a website used frequently by the group.