Earlier, the US military announced a marine with the 1st Marine Expeditionary Force died in action in the central province of al-Anbar.
The names of the US troops slain were withheld pending notification of the family, and no other details were given on the car bombing.
The news came after US military officials in Iraq announced they had thwarted an attempted prison escape at Camp Bucca, discovering a 180-m tunnel leading out of the main prison facility for detainees in Iraq.
The tunnel reached beyond the compound fence, but no one had yet escaped, said Major Flora Lee, a spokeswoman at the US Army's Combined Press Information Centre in Iraq.
She did not know when guards discovered the tunnel.
Camp Bucca holds 6049 detainees, nearly two-thirds of all those in Iraq, Lee said. Situated near the southern city of Umm Qasr, it is one of three detainee facilities in Iraq.
"There have been a few other attempts at digging a tunnel but nothing of this size"
Major Flora Lee,
US Army spokeswoman
A bucket cut from a water container and a shovel made of tent material were used to dig the tunnel, Lee said. The opening was under a floorboard of the compound and was concealed with dirt.
The authorities in charge of the compound realised a tunnel was under way after they found dirt in latrines and other places, Lee said. It may have been the most extensive effort aimed at a mass escape.
"I'm not aware of any other instances where this has happened," Lee said. "There have been a few other attempts at digging a tunnel but nothing of this size."
US guards fired on prisoners during a riot at Camp Bucca on 31 January, killing four detainees and injuring six others.
Also on Saturday, a senior Iraqi Defence Ministry official said Iraqi troops backed by US forces detained 121 suspected fighters and uncovered a massive weapons cache during a joint raid south of Baghdad.
The official, speaking on customary condition of anonymity, said the operation at the town of Musayyib turned up hundreds of Kalashnikov rifles and rocket-propelled grenades, as well as car bombs, machine guns, rockets, mortar rounds and other munitions.
Fighters are suspected to have
been planning an attack in Karbala
Gunfire broke out during the raid, but there were no injuries to any US troops or Iraqi security forces, the official said. There was no word on any casualties among the suspected fighters' ranks.
Some of the suspected fighters planned to attack Shia Muslims expected in the coming days to head to an annual religious celebration in the nearby city of Karbala, the official said.
A US military spokesman had no immediate information on any raid at Musayyib, 60km south of Baghdad, and an interim Iraqi Defence Ministry spokesman was not immediately able to confirm the operation.