Story Highlights• NEW: President Bush says he does not consider report credible
• Gunfire found to be most common killer of Iraqis; car bombings on the rise
• Study says 2.5 percent of population killed since war; death toll rising each year
• Coalition forces blamed for 31 percent of deaths since 2003 invasion
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BALTIMORE, Maryland (CNN) -- War has wiped out about 655,000 Iraqis or more than 500 people a day since the U.S.-led invasion, a new study reports.
Violence including gunfire and bombs caused the majority of deaths but thousands of people died from worsening health and environmental conditions directly related to the conflict that began in 2003, U.S. and Iraqi public health researchers said.
"Since March 2003, an additional 2.5 percent of Iraq's population have died above what would have occurred without conflict," according to the survey of Iraqi households, titled "The Human Cost of the War in Iraq." (Watch as the study's startling results are revealed -- 1:55 )
The survey, being published online by British medical journal The Lancet, gives a far higher number of deaths in Iraq than other organizations. (Read the full report -- pdf)
The report's release came as nearly four dozen Baghdad civilians became casualties in another day of bombs and gunfire. (Full story)
President Bush slammed the report Wednesday during a news conference in the White House Rose Garden. "I don't consider it a credible report. Neither does Gen. (George) Casey," he said, referring to the top ranking U.S. military official in Iraq, "and neither do Iraqi officials."
"The methodology is pretty well discredited," he added. (Watch Bush dismiss the report -- 1:33 )
Ali Dabbagh, an Iraqi government spokesman, said in a statement that the report "gives exaggerated figures that contradict the simplest rules of accuracy and investigation."
Last December, Bush said that he estimated about 30,000 people had died since the war began.
When pressed whether he stood by that figure Wednesday, he said, "I stand by the figure a lot of innocent people have lost their life. Six hundred thousand -- whatever they guessed at -- is just not credible."
Researchers randomly selected 1,849 households across Iraq and asked questions about births and deaths and migration for the study led by Gilbert Burnham of Johns Hopkins Bloomberg School of Public Health in Baltimore, Maryland. The Center for International Studies at Massachusetts Institute of Technology cooperated.
They extrapolated the figures to reflect the national picture, saying Iraq's death rate had more than doubled since the invasion.
On Wednesday, Burnham defended his team's methodology, saying it was the standard used in developing countries to survey for HIV and other major health issues he said. In 87 of the interviews conducted, the researchers asked for death certificates, and people were able to produce one 92 percent of the time, he said.
In 13 percent of the interviews, the researchers had forgotten to ask for certificates, he said. (Watch Military and civilian experts question the methodology -- 1:45 )
The report said that Iraqis "bear the consequence of warfare" and compared the situation with other wars: "In the Vietnam War, 3 million civilians died; in the Congo, armed conflict has been responsible for 3.8 million deaths; in East Timor, an estimated 200,000 out of a population of 800,000 died in conflict.
"Recent estimates are that 200,000 have died in Darfur [Sudan] over the past 31 months. Our data, which estimate that 654,965 or 2.5 percent of the Iraqi population has died in this, the largest major international conflict of the 21st century, should be of grave concern to everyone."
The researchers estimated that an additional 654,965 people have died in Iraq since the invasion above what would have been expected from the pre-war mortality rate. They did not ask families whether their dead were civilians or fighters. (Read the report's appendix, including methodology and charts -- pdf)
Violence claimed about 601,000 people, the survey estimated -- the majority killed by gunfire, "though deaths from car bombing have increased from 2005," the study says.
The additional 53,000 people who are believed to have been killed by the effects of the war mostly died in recent months, "suggesting a worsening of health status and access to health care," the study said. It noted, however, that the number of nonviolent deaths "is too small to reach definitive conclusions."
Other key points in the survey:
Burnham said the confidence interval of the data put the range of the number of deaths between 400,000 and 900,000. He suggested the media should not get too focused on the 655,000 number.
Professionals familiar with such research told CNN that the survey's methodology is sound.
It has been very difficult to pin down fatality numbers during the Iraq conflict.
The private British-based Iraq Body Count research group puts the number of civilian deaths at between 43,850 and 48,693. Those figures are based on online media counts and eyewitness accounts.
"The count includes civilian deaths caused by coalition military action and by military or paramilitary responses to the coalition presence (e.g. insurgent and terrorist attacks)," the group's Web site says. "It also includes excess civilian deaths caused by criminal action resulting from the breakdown in law and order which followed the coalition invasion."
The latest estimates were released less than a month ahead of U.S. midterm elections that could change the balance of power in the House and Senate, now controlled by Republicans.
CNN's Jomana Karadsheh contributed to this report
An Iraqi weeps as his slain relative's coffin is searched for explosives during a funeral in Najaf.
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