The military also said on Friday that 13 marines, 11 of them women, were wounded in the attack on a military convoy in Falluja on Thursday night.
The troops were returning to their base in the city after manning checkpoints around it, where female soldiers were deployed to search women entering and leaving.
In a statement posted on the internet that could not be verified, an al-Qaida-linked group in Iraq claimed responsibility for the attack.
Six marine soldiers were killed and two Humvee vehicles were destroyed, the group's statement claimed.
Al-Sistani aide killed
In a separate incident, one of Ayat Allah Ali al-Sistani's senior aides, Samir Baghdadi, and two of his guards were killed in Baghdad.
Witnesses said the victims were heading for the town of Karbala when the vehicle they were travelling in came under fire.
Aljazeera has also reported that Iraqi forces have displayed a large collection of weapons, including mortar shells confiscated south of Baghdad on Friday.
In the northern city of Mosul, four Iraqis, including a police officer, were killed and five others wounded in three separate attacks late on Thursday and early on Friday, police Brigadier-General Wathiq Muhammad said.
Three policemen were also wounded on Friday in a roadside bombing that targeted their convoy in the northern oil city of Kirkuk.
More than 600 checkpoints have
been set up around Baghdad
At least 35 people were killed and about 100 wounded in coordinated car bomb attacks in central Baghdad on Wednesday and Thursday.
According to Lieutenant-Colonel Michael Pryor of the US 3rd Infantry Division's Task Force Baghdad, attacks are becoming less coordinated and less successful. "We think the intent is just to cause a lot of noise to make it seem like there are a lot of activities," he said.
The sound of gunfire and explosions is still common in the capital, however, with almost daily car-bomb attacks and drive-by shootings killing more than 300 around the country in June, many in the Baghdad region.
More than 600 checkpoints, set up around the capital, were to serve as a cordon to keep fighters out, Iraq officials had said when they kicked off Operation Lightining, at the end of May.
Pryor said: "I think there will be a lot of small attacks in the next couple of weeks with the (Iraqi) prime minister going to the United States."
US military says they are
expecting more violence
Pryor said: "They're trying to influence the international population through the media that Iraq may not be as secure as everyone makes it out to be."
Iraqi Prime Minister Ibrahim al-Jaafari was in Washington on Friday to meet President George Bush and help him ease increasing American concern about the Iraq war.
In a separate incident in Baiji, an oil pipeline, carrying crude from Kirkuk to Baghdad, was attacked. The fire has been raging for two days.
In another development, sources in the Iraqi police said they found six beheaded bodies in Khan Bani Saad district, south of Baquba.
Meanwhile, the deputy head of the Sunni Waqf (endowment), Shaikh Ahmed Abd al-Ghafour al-Samarraei, said the recent military operations in Baghdad followed by the bombings was an attempt to incite sectarian violence.
The Iraqi people have been victimised by US arrogance, random detentions and by assassinations, al-Samarraei said.
The Association of Muslim Scholars (AMS), on its part, has denounced the bombings in the al-Karada area of Baghdad. A spokesman for association said the AMS saw the Shia as Iraqi brothers - bearing the same citizenship and are brothers in faith – having the same religious belief as the Sunnis.