The deaths announced on Monday brought the number of US service members killed so far this month to 13 - nearly half the number who died in the whole of March.
Three more Americans - two marines and a sailor - were missing in the Sunday accident in which a truck overturned near Asad air base, a US statement said. All the dead were marines, the statement added.
It gave no reason for the accident except that it was not due to hostile fire. Heavy rains fell over the remote area over the weekend.
Also on Sunday, three marines and a sailor were killed by "hostile fire" in Anbar province, which also includes the Asad base, the military said. No further details, including the precise location, were released.
Besides the US soldiers, violence claimed the lives of at least eight Iraqis on Monday.
Two car bombs exploded in Baghdad, killing a bystander and wounding half a dozen others.
One detonated in eastern Baghdad's Sadr City, the other in the central district of Karrada, both mostly Shia areas.
The Sadr City explosion killed at least one civilian and wounded seven others, including a nine-year-old boy. Two were wounded in Karrada.
The intended targets were not known, police said.
In Basra, six people were killed in a drive-by shooting in a market area, including a navy officer, two policemen, two workers at an electrical plant, and a boy, police said.
An Iraqi soldier secures the site
of Monday's blast in Sadr City
Basra, Iraq's second largest city, is 550km southeast of Baghdad.
Police also reported on Monday that a gang of armed men killed a family in their home late on Sunday.
The police said that in Baghdad's Dura district, four men charged into a Shia home, lined up a brother, two sisters, and an uncle against a wall and shot them.
The father of the family, a grocery shop owner, had been killed six months earlier by armed men in the same neighbourhood, one of Baghdad's most dangerous. The mother was visiting relatives when the attack occurred.
In further incidents, blasts damaged several buildings in the town of Buhriz, 60km northeast of Baghdad, including a barber shop and a grocery in the market district.
Police separately discovered two corpses in eastern Baghdad, one in Mashtal that was handcuffed and shot in the head, another in Baladiyat that was strangled and covered with bandages.
In Dura, drive-by shooters killed a police captain outside his home late on Sunday, police said. In the northern city of Kirkuk, attackers killed a Sunni cleric on Monday as he headed home from the al-Quds mosque, police said.
Four members of a Shia family
were gunned down in Baghdad
Also reported on Monday were two incidents in Baiji. In the first, two soldiers were killed and three were wounded when armed men attacked their patrol, police said.
In the second, a policeman was killed and two others wounded when their patrol was attacked.
The Iraqi government said the Iraqi army arrested 12 fighters near Mosul on Sunday.
Meanwhile, in the north, the regional government of Kurdistan released the Kurdish writer Kamal Karim just a week after he received an 18-month sentence for articles on a Kurdish website that accused one of the region's leaders of corruption, said Mohamed Khoshnaw, a government spokesman.