The World Health Organisation said a large-scale household survey showed an estimated 151,000 Iraqis were violently killed since the US-led invasion of their country in March 2003 to the middle of 2006.
WHO said the findings, published in the New England Journal of Medicine, were the result of a wider family health survey to provide the Iraqi government with the basis to develop and update its health policies and services.
"Assessment of the death toll in conflict situations is extremely difficult and household survey results have to be interpreted with caution," said Mohamed Ali, a WHO statistician Wednesday.
"However, in the absence of comprehensive death registration and hospital reporting, household surveys are the best we can do."
WHO said the estimate was based on interviews in 9,345 households in nearly 1,000 neighbourhoods and villages across Iraq, placing the number of violent deaths between 104,000 and 223,000. WHO chose to focus on the middle figure of 151,000 to illustrate its survey.
Naeema al-Gasser, the WHO representative in Iraq, said the estimate was three times higher than the death toll calculated through careful screening of media reports by the Iraq Body Count project, a joint effort of Baghdad and organisations trying to determine the number of people killed since 2003.
But the researchers also said the survey's estimate was four times lower than a small-scale household survey conducted in 2006.
Some homes cannot be visited because of high levels of insecurity and some residents moved home in times of conflicts.
"The study found that violence became a leading cause of death for Iraqi adults after March 2003 and the main cause for men aged 15-59 years," WHO said in a statement at UN headquarters in New York.
It said on average 128 Iraqis have been killed a day in the first year of the war, 115 deaths a day in the second year and 126 in the third year.
"More than half of the violent deaths occurred in Baghdad," WHO said.