Gregg Zoroya, USA TODAY

More than 600,000 Iraqis have died by violence since the U.S.-led invasion in 2003, according to a study released Wednesday by researchers at Johns Hopkins University.

The figure is based on surveys of households throughout most of the country. It vastly exceeds estimates cited by the Iraqi government, the United Nations, aid and anti-war groups, and President Bush.

The new estimate was immediately challenged by the Pentagon. Lt. Col. Mark Bellesteros, a Pentagon spokesman, said the Iraqi government "would be in a better position ... to provide more accurate information on deaths in Iraq."

Frederick Jones, a spokesman for the National Security Council said "many experts" found that a 2004 study by the same group "wildly inflated the findings." That study said the war had caused 100,000 Iraqi deaths.

"This study appears to be equally flawed," he said. The new study said the deaths have resulted from coalition military activity, crime and religious violence.The Iraqi government dismissed the Johns Hopkins estimate. The toll in the report "exceeds the reality in an unreasonable way" and the report "gives inflated numbers in a way that violates all rules of research and the precision required of research institutions," Iraq spokesman Ali al-Dabbagh said in a statement.

"These numbers are far from the truth," al-Dabbagh said.

The Iraqi government has never given an official number of people killed since the U.S. invasion.

Iraq's Health Ministry has estimated 50,000 violent deaths since the war began, through June. Last December, President Bush put the figure at 30,000. The Brookings Institution, a Washington think tank, estimated the death toll at 60,000.