The attack on Thursday, between Baquba and Kanaan, some 50km north of the capital, was carried out by unidentified armed men in a car.
In Baghdad itself, the owner of a tile-making factory and two of his sons were shot dead overnight at their home by an armed group posing as policemen, an Interior Ministry official said.
His wife was wounded and another son kidnapped in the attack in the New Baghdad district, in the southeast of town.
Also in Baghdad, an unidentified armed group fire on a pickup truck carrying six security guards from the Ministry of Immigration, killing four and wounding two, an Interior Ministry official said. The armed group escaped.
Meanwhile, a roadside bomb hit a US patrol of Humvees in southern Baghdad, killing one soldier and wounding six, an official said.
Pipelines and oil facilities have
been targets since April 2003
The explosion occurred at 10.30pm (1830 GMT) on Wednesday in the Dora section of Baghdad, said Sergeant 1st Class David Abrams of the US Army.
The residential area of the capital has been the site of many attacks by fighters against American forces and Iraqi police.
Abrams said he could not immediately identify the victims of the attack or their unit.
Elsewhere, a bomb damaged an oil pipeline near the oil-rich city of Kirkuk, sending plumes of black smoke and fire up into the air, officials said on Thursday.
The bomb, which exploded late on Wednesday, was placed beneath the aboveground pipeline, which connects the Bay Hassan oil fields with Kirkuk in northern Iraq, said police Brigadier Sarhad Qadir.
Officials in Iraq's Northern Oil Company said the heavily damaged pipeline would be repaired within five days.