The high death rate is mainly due to violence, with many of the victims women and children, according to US public health experts.
"Making conservative assumptions, we think that about 100,000 excess deaths or more have happened since the 2003 invasion of Iraq," researchers from Johns Hopkins' Bloomberg School of Public Health in Baltimore, Maryland, said in a report published online by The Lancet medical journal on Thursday.
"Violence accounted for most of the excess death and air strikes from coalition forces accounted for the most violent deaths," the report added.
The new figures, based on surveys carried out by researchers in Iraq, are much higher than earlier estimates based on thinktank and media sources, which put the Iraqi civilian toll as high as 16,053 and military fatalities as high as 6370.
By comparison, 852 US troops were killed in combat or attacks and 258 died in accidents or incidents not related to fighting, according to the Pentagon, bringing the total to 1110 since the 20 March 2003 invasion.
Mainly women and children were
killed, the report said
More than 8000 US soldiers have been wounded in the same time period.
"The risk of death from violence in the period after the invasion was 58 times higher than in the period before the war," Les Roberts and his colleagues said in the report which compared Iraqi deaths during the 14.6 months before the invasion and the 17.8 months after it.
He added that violent deaths were widespread and were mainly attributed to US-led forces.
"Most individuals reportedly killed by coalition forces were women and children," Roberts said.