A prominent Sunni tribal leader and his son and seven bodyguards have been killed in the Iraqi capital in an attack that has raised fresh questions about the role of armed groups.

Officials said on Saturday that Sheikh Qassem Sweidan al-Janabi was killed in Sadr City, a mainly Shia district in east Baghdad, after his convoy was ambushed.

Janabi, a moderate Sunni, was found shot in the head with his hands tied behind his back.

His son was killed by a bullet to the chest while most of his seven bodyguards were shot in the head.

The sheikh's nephew, politician Zayed al-Janabi, was also detained during the ambush but released unharmed.

No group claimed responsibility for Saturday's attack but Saleh Mutlaq, Iraq's deputy prime minister, called for curbs on Shia armed groups.

"We must get rid of the militias, and weapons must be in the hands of the state," he said before the men were buried.

Adnan al-Janabi, a member of parliament from the same tribe, said Sheikh Qassem, who was a particularly prominent figure in the religiously mixed areas south of Baghdad, was a key player in efforts to combat sectarianism.

He called on the government to "take a stand against the militias", saying that the assassination was evidence that some people in Iraq were "working to sabotage the achievements of the state".

Nickolay Mladenov, the top UN envoy in Iraq, also called for the killers to be brought to justice.

Shia armed groups operate freely in many areas and have spearheaded the battle against the Islamic State of Iraq and Levant (ISIL) group.

In January, Sunni politicians and tribal chiefs in the eastern Diyala province accused Shia fighters of killing at least 70 unarmed civilians who had fled from ISIL-controlled region.

Source: Al Jazeera and agencies