The US occupation soldiers were killed in two attacks near the western city of Fallujah.
A car laden with explosives ploughed into a military checkpoint west of Baghdad and exploded, killing three US soldiers and wounding six, the US military and witnesses said.
Three US soldiers "were killed and six were wounded when a vehicle-borne IED (improvised explosive device) detonated at Khaldiyah," 95km west of Baghdad, a US military spokesman said.
Khaldiyah lies between Fallujah and Ramadi, an axis of fighting against the US-led occupation.
Two US pilots were killed late on Friday when their helicopter came down near the northern city of Kayyarah, but it was not clear whether hostile fire was involved.
The deaths brought to 239 the number of US occupation soldiers killed in action since President George Bush declared major combat over last May.
Seven Iraqis killed
Elsewhere, at least four Iraqis were killed and around 38 injured when a powerful car bomb exploded in Samarra, 125km north of Baghdad, and three Iraqi policemen died in two drive-by shootings and a bombing in Baghdad and the northern cities of Mosul and Kirkuk.
Smoke billows from burning
vehicles in Samarra
The car bomb went off in a crowded square in front of the Samarra courthouse, witnesses said, with a force that threw the vehicle high in the air, leaving a two-metre crater and wrecking other cars and shattering court windows.
US military police First Lieutenant Alexis Marks said a US patrol was just 75 metres from the courthouse when the blast occurred.
"The blast was so powerful it rocked the armoured vehicles," she said. "I was not directly facing that way. It knocked me off my feet."
An Iraqi official said the target might have been members of the district council who were to meet a delegation of foreign election officials in a nearby building at the time of the blast.
UN team in Iraq
The attacks came as a two-man UN security team began a mission to Iraq, ending a general withdrawal of UN personnel three months ago ordered by Secretary General Kofi Annan after a spate of deadly bomb attack on UN headquarters in Baghdad.
US soldiers inspect the debris of
a vehicle in Samarra
Efforts to lure the UN into a renewed role in the planned transfer of power from the US-led occupiers to Iraqis had been boosted earlier by a call from top Shia cleric Ali al-Sistani to halt protests against the plans.
Meanwhile, US Secretary of State Colin Powell conceded it was an "open question" whether Iraq had weapons of mass destruction prior to the US-led invasion, but insisted pre-war intelligence had been correct about Baghdad's intention to develop them.
Powell was responding to comments by David Kay, who stepped down on Friday from leading the US hunt for Iraq's illicit weapons, saying he thought no such arsenal had existed at the start of the war in March last year.
"What is the open question is how many stocks they had, if any, and if they had any, where did they go? And if they didn't have any, why wasn't that known beforehand?" Powell told journalists travelling with him from Washington to the Georgian capital Tbilisi.