The dead included Riyadh al-Sammarrai, the head of a Sunni volunteer force that has been co-operating with US-led forces against al-Qaeda in Iraq.
In another incident in Baghdad, armed men in five cars kidnapped between eight and 10 neighbourhood patrol volunteers in the capital's northern Shaab district.
Before the new year, Osama bin Laden, the al-Qaeda chief, vowed to strike the volunteers, who are funded by US forces.
Other blasts in Baghdad killed five people on Monday.
These included a bomb hidden in a market cart that killed four in the central Karrada district and a pair of roadside bombs that killed a civilian and wounded two policemen in southern Jadiriya district.
|Monday's bombings were the latest in a series|
of attacks on US-allied Sunni groups [Reuters]
Police said they found five bodies of men handcuffed, blindfolded and shot in the head in Diyala province.
In Samarra, assailants killed a member of a neighbourhood patrol working as a carpenter inside his shop.
Elsewhere, a large fire erupted at the Baiji oil refinery, a member of the Oil Protection Force guard unit said.
"The fire erupted in a unit in the refinery. It is a huge fire. Firefighting trucks are trying to extinguish the blaze," he told Reuters news agency.
There were no immediate reports of casualties.
Two bodies were found in the eastern part of Mosul, one of them handcuffed and blindfolded, police said.
The previous day, police said three churches and a monastery were hit in attacks on Christian religious buildings in the city.