The soldiers were killed on Monday near Haditha, a town on the Euphrates river 200km (120 miles) northwest of Baghdad, said Lieutenant-Colonel Steve Boylan, a US military spokesman.
The al-Qaida-linked Ansar al-Sunna said in an internet statement on Wednesday that it had killed eight US marines in western Iraq. Ansar al-Sunna said it had killed some by "slitting their throats," while others were shot.
The statement could not be verified, and it
was not immediately clear whether the marines were killed in a single attack or if they died in separate clashes with fighters.
The deaths bring to just over 1800 the number of US soldiers who have died since the start of the war in Iraq in March 2003.
In the past month, nearly 60 have died, including five who were killed in roadside bomb attacks in Baghdad at the weekend.
There have been frequent attacks in the area around Haditha in recent months. US forces have launched at least two major offensives to try to quell uprising in the region, one of the most violent in the country.
Haditha is in the western al-Anbar province, which also includes the cities of Falluja and Ramadi, and has become the heartland of the two-year-old Sunni Arab-led revolt.
At least 23 people were killed in ongoing violence across Iraq on Tuesday as members of parliament discussed issues holding up the completion of the country's new constitution.
A powerful blast shook central Baghdad when a car bomber blew himself up close to a US military convoy. Four people were killed and 23 others were wounded, including four women, medics said.
Wounded civilians are brought
to hospital in Baghdad
"We have received four bodies and 23 other people who are wounded from the car bomb," said a doctor at Ibn al-Nafis hospital.
One US Humvee was set ablaze and 14 other vehicles were damaged by the blast, which occurred at around 1pm (0900 GMT), an AFP correspondent reported. There were no immediate reports of US casualties.
"We were 20m away when the car bomb went off and me and my team carried 15 wounded civilians," said policeman Hussein al-Musawi, his shirt soaked with blood.
The latest car bombing came as parliament worked on a draft constitution that MPs have vowed will be ready by 15 August, on time for a scheduled mid-October referendum.
In another incident, a civilian was killed and five wounded, four of them policemen, when a suicide car bomber attacked a police patrol in the centre of Baquba, 60km northeast of Baghdad, police said.
Four Iraqi soldiers were killed when a bomb hidden inside a dead dog hit an army patrol in the northeastern Balad town. Five soldiers were also wounded.
Three other people working at a US base in the northern town of Baiji were killed when the bus they were travelling in was attacked by armed attackers, while a construction worker was shot dead, also in Baiji.
"We were 20 meters (yards) away when the car bomb went off and me and my team carried 15 wounded civilians."
Hussain al-Musawi, Iraqi policemen
An engineer was gunned down in the northern town of Dhuluiyah, while a man was killed in a Baghdad bookshop when a bomb reportedly hidden in a suitcase blew up, witnesses said.
The body of an Iraqi soldier was found in Samarra, while that of a policeman was found in Al-Dawr, 150km north of Baghdad.