Captain Bill Coppernoll, spokesman for the Tikrit-based US 1st Infantry Division, said the attackers fired from two cars at several buses carrying Iraqis working for the US-led forces that had stopped in northern Tikrit, 130km north of Baghdad.
The attack occurred at about 8.30am (0530 GMT) on Sunday.
"Two vehicles opened fire on civilian buses that had stopped to let the workers off, who were employed by coalition forces," Coppernoll said. "They were going to work."
An Iraqi journalist, Khalid Abd Allah, told Aljazeera that 15 others were injured in the attack.
Fighters opposed to the US-led government regularly target people they see as collaborating with enemy forces.
Elsewhere on Sunday, a bomber drove into an Iraqi National Guard checkpoint in Baiji, about 120km further north from Tikrit, detonating his explosives-packed vehicle, and killing three Iraqi National Guard soldiers and wounding 18 at about 9.30am (0630 GMT), Coppernoll said.
The bomber drove the vehicle
into an Iraqi police checkpoint
Gunmen opened fire on the checkpoint after the blast, Coppernoll added.
Anti-US fighters also attacked patrolling Iraqi National Guard officers near Samarra, 95km north of Baghdad, early on Sunday, killing one Iraqi soldier and wounding four.
Aljazeera has also learnt that clashes erupted on Sunday between anti-US fighters and the Iraqi National Guard in the al-Dura district of southern Baghdad. The guards launched a search campaign in the area later, to track down the fighters.
Similiar clashes also occurred in in al-Adhamiya district in central Baghdad. Details of any casualties are not yet available.
US soldiers killed
Violence this weekend has already resulted in the deaths of two US soldiers when their patrol came under attack in the northwestern city of Mosul. Four soldiers were wounded in the incident.
The soldiers, part of the US Task Force Olympia based out of Mosul, were attacked at about 3pm (1200GMT) on Saturday, according to a military statement.
Clashes in Mosul have intensified
during recent weeks
The wounded soldiers were evacuated to a Mosul military hospital for treatment.
Another three US soldiers were wounded while on patrol on Saturday, but returned to duties, after their Humvee was attacked by a roadside bomb near Miqdadiya, about 90km north of Baghdad, according to 1st Infantry Division spokesman Master Sergeant Robert Powell.
Powell said a truck travelling in a US logistics convoy was struck on Saturday by another improvised explosive device near Baiji, 145km south of Mosul, killing the driver who was a third-country national. No further details were available.
Kurdish militiamen attacked
Also on Saturday, a bomber exploded his vehicle alongside a bus carrying Kurdish militiamen, killing at least nine people and wounding nine more, Kurdish officials said.
Fighters opposed to the US-led government continued their campaign against the Iraqi National Guard on Saturday, killing one officer and wounding another in Abayach, near Balad, 80km north of Baghdad, Powell added.
Mosul has witnessed a recent upturn in conflict targeting American soldiers, Iraqi security forces and Kurdish groups as anti-US forces intensify efforts to destabilise the country before planned 30 January national elections.
Hundreds of fighters are believed to have flocked there last month after US-led forces stormed the city of Falluja.
On Friday, armed men tried to seize four Mosul police stations, but were repelled, the US military said.
About 70 fighters also ambushed a US patrol with roadside bombs, rocket-propelled grenades and small-arms fire.
After regrouping, US and Iraqi forces struck back, killing more than two dozen fighters, the military said.