Elsewhere, three Iraqis, including a woman, were killed in a string of attacks, police said.
Four men driving in a four-wheel-drive vehicle, which are often used by foreign contractors, were hit by a roadside bomb and then gunfire in Ashaki, south of the Sunni town of Samarra, said Lieutenant-Colonel Hamid Muhammad on Monday.
The bodies were picked up by other cars in the dead men's convoy, he added.
The US military said it had no information on the incident.
Earlier, an Iraqi truck driver leaving a US military base near the town of Yethrub, north of Baghdad, was shot dead, said Lieutenant-Colonel Ali Abd Allah.
Clashes between fighters and US
forces have continued in Falluja
An Iraqi translator for the US military was also shot dead by unknown attackers near Salman Pak, said Major Hamid Abd Allah of the Salman Pak police.
Meanwhile, an Iraqi woman was killed and three civilians wounded when a roadside bomb exploded on the road between the cities of Samarra and Falluja, said police Lieutenant-Colonel Mahmud Muhammad.
In Falluja, fighters opposed to the presence of foreign troops in Iraq clashed with US troops in the south and eastern parts of the city.
A US official said fighting was continuing in the battered city, preventing the return of many Falluja residents.
In the main northern city of Mosul, US and Iraqi forces searched a mosque and arrested its main cleric, said a religious leader.
US soldiers and Iraqi guardsmen raided the Ahmad Ismail mosque in northern Mosul late on Sunday and detained Shaikh Bashar Awad, said fellow mosque member Shaikh Natham Basil.
Awad belongs to the Association for Muslim Scholars (AMS), an influential group that has vowed to boycott January's elections.
US soldiers detained a man linked
to the AMS
A US army spokesman said the military had no record of the raid but said the army had searched the al-Sabrin mosque in southeastern Mosul overnight.
They were looking for a cleric "responsible for inciting anti-Iraqi rhetoric" but could not find him, he added.
In Baghdad, two mortar shells struck inside a Sunni Muslim mosque complex on Sunday, wounding four guards, one of them seriously, a religious leader said.
The mortars landed in the garden at the Imam Ibin Taimiya mosque, damaging the walls and shattering windows, said Shaikh Zakhariya Abu Yahya.
"It's the bad security situation in Iraq. I can't accuse anyone," said Abu Yahya.
The senior cleric at Imam Ibin Taimiya, Shaikh Mahdi al-Sumaiydai, was arrested by US troops in mid-November after soldiers allegedly found weapons inside the compound.