About 13,000 Iraqis, including as many as 4,300 civilians, were killed during the major combat phase of the Iraq war, according to a US research group.
US air raids targeted military and government facilities
It said the estimates were based on US combat data, battlefield press reports, and Iraqi hospital surveys.
Despite the advent of precision weapons, more civilians died in the latest conflict than in the 1991 war, the group suggests.
The US military has published no details on Iraqi deaths in either war.
The study by the Project on Defense Alternatives (PDA) covers the period from 19 March to the end of April.
It provides ranges of casualty levels, rather than specific estimates.
It says that as few as 11,000 Iraqis may have been killed in the war, or as many as 15,000 - the 13,000 being the mid-point between the two figures.
According to the PDA, approximately 30% of the victims were "non-combatants" - defined as civilians who did not take up arms.
These are "working" figures, which could change as new information becomes available, the group makes clear.
The study estimates the Iraqi dead during the 1991 Gulf War at some 3,500 civilians and between 20,000 and
The report says the Iraqi War fatalities point to the "paradox of the New Warfare".
"One premise of the 'new warfare' hypothesis is that precision technologies and new warfighting techniques now allow the United States to wage war while incurring dramatically fewer casualties - especially civilian casualties.
Many Iraqi civilians were caught in the bombing
"Although Operation Iraqi Freedom was supposed to exemplify the new warfare, it provides no unambiguous support for the hypothesis regarding civilian casualties," the author writes.
However, the report adds, "the power and promise of the new warfare is evident in having achieved so much more in the 2003 war than in the 1991 war, while incurring a comparable or lower cost in lives".
The PDA is a think tank based in Cambridge, Massachusetts.