Marine Lieutenant Lyle Gilbert said the blast occurred at about 7am (0400 GMT) at an Iraqi police post near al-Asad camp, about 180km north-west of Baghdad, between the towns of Hiyt and Haditha. He said there were no US casualties.
Hospital staff in Haditha earlier put the toll at nine and said they had received more than 50 casualties.
Also on Saturday, a bomber blew up his vehicle near a checkpoint manned by Iraqi National Guards north of Baghdad, killing four guards, an Iraqi National Guard officer said.
He said six guards were also wounded when the bomb exploded in the village of Ishaqi, near the city of Samarra, 100km north of Baghdad.
The US military had no immediate word on the blast.
Two mortar rounds have been fired in central Baghdad, killing two civilians and wounding one, witnesses said.
A telephone worker said he had taken all three casualties to hospital in his car after one mortar bomb hit the Karrada commercial district. A donkey was also killed.
Witnesses said the other mortar round appeared to land in the fortified Green Zone, which houses Iraqi government offices and the US and British embassies. There were no reports of casualties or damage in that blast.
In a separate incident two Turkish truck drivers were killed and two wounded on Saturday, near the northern Iraqi city of Mosul, police and hospital officials said.
Truck drivers are often targeted
while negotiating the highways
One of the wounded drivers told Reuters they came under fire from gunmen while they were transporting canned juice to the US military in Mosul.
Many Turkish truck drivers have been attacked or taken captive along the highway from Turkey to Iraq.
Earlier five US soldiers were injured when a roadside bomb exploded in the path of their vehicle in western Baghdad, a US military spokesman said, adding that none of the troops were badly hurt.
"This morning a Bradley was struck by an IED (improvised explosive device). Five soldiers were injured," said the spokesman, Major Philip Smith.
The area around the Bradley had been cordoned off and the vehicle would be recovered later, Smith added.
An Iraqi journalist told Aljazeera a US military vehicle was seen ablaze at the entrance to the highway that leads to Baghdad's international airport - one of the most dangerous roads in Iraq for roadside blasts and ambushes.
Overnight, US warplanes bombed the Iraqi city of Falluja, killing two people and wounding three.
Hospital staff said they had received the casualties after what residents said was an air strike on the southern edge of Falluja at about midnight (2100 GMT) on Friday.
US army said a senior member of
al-Zarqawi network is in custody
Residents said US forces detained at least 17 people in outlying areas of the city in raids before dawn.
The US military also said it had captured a "senior leader" in the network allegedly run by al-Qaida-linked Abu Musab al-Zarqawi, during the raid.
A statement said a lieutenant of al-Zarqawi's network had been captured in a raid at 1.30am (2230 GMT) on Friday in what it called a safe haven in southern Falluja.
"The individual targeted and captured today was recently assessed to be a relatively minor member of the Zarqawi network, according to intelligence sources," the statement said.
"However, due to a surge in the number of Zarqawi associates who have been captured or killed by (US) strikes and other operations, the member had moved up to take a critical position as a Zarqawi senior leader," it said, without identifying the man or stating his nationality.
"There were also five other terrorists detained during this raid," the statement added.
Aljazeera has also learned that US troops arrested Mahmud al-Jarassi, deputy governor of Falluja, when he arrived in the city on Saturday morning.
Determined to regain control of the city, which has become a no-go zone for the US army, more than 1000 joint US and Iraqi forces have encircled the area since last Friday.
Iraqi Prime Minister Iyad Allawi last week ordered Falluja residents to hand over al-Zarqawi or face invasion. Fallujans have denied that al-Zarqawi is holed up with them.
Humanitarian agencies have raised concerns for the welfare of residents in Falluja, which is being hit by US air raids almost every night.