An undetermined number of Iraqi civilians have also been wounded.
"Three Task Force all American soldiers were killed and six were wounded when a vehicle loaded with explosives detonated at Khaldiyah," a US military spokesman said.
Locals said the vehicle blew up at a military post in the town, 95 km west of Baghdad.
The blast in Khaldiyah tops a bloody day of resistance attacks across Iraq, that left another 10 people dead including two US soldiers and three Iraqi policemen.
US military said the two soldiers were killed when their convoy hit an explosive device north of the restive town of Fallujah.
The latest US deaths brought to 509 the number of American service members who have died since the United States and its allies launched the Iraq war 20 March.
Most of the deaths have occurred since President George W. Bush declared an end to major combat 1 May.
Another five Iraqis were killed in a massive blast in the town of Samarra.
Three Iraqi police officers also died on the day in separate attacks.
An Iraqi traffic policemen died and three others were wounded when unknown gunmen opened fire from a speeding car in Baghdad.
Another Iraqi policeman died in the northern city of Mosul when his vehicle came under attack.
Yet another policeman was killed and two wounded when an explosive device went off in th oil city of Kirkuk
Meanwhile, the blast in the centre of Samarra outside a crowded courthouse, 125 km north of Baghdad in the Sunni heartlands, left a trail of devastation, including wrecked cars and broken window panes.
A US military spokesman said the bomb was hidden in a white Toyota car parked in front of the courthouse in an area crowded with people at the time.
The force of the blast left a two metre crater in the ground and killed at least four people according to Aljazeera's correspondent in the town.
Locals said six Iraqi policemen were among the more than 30 wounded, and included several US troops.
Resistance fighters have regularly targeted Iraqi police, often in car bomb strikes on police stations. Other Iraqis seen as cooperating with occupation forces have also been hit.
Saturday's blast occurred just as a two-man UN security team began a mission in Iraq, ending a three-month absence of UN international staff since a spate of deadly attacks last year.
The team was charged only with opening up channels of communication with the coalition, a spokesman said.