"I don't consider it a credible report," Mr Bush told a White House press conference in response to a study published in the medical journal The Lancet. "Neither does General Casey, neither do Iraqi officials," he added, referring to George Casey, the top US general in Iraq.

The Iraqi government said the report's death toll was "inflated" and "far from the truth", but did not give its own figure for the deaths.

Mr Bush, who last December suggested there may have been 30,000 civilian deaths in Iraq, would not give a figure today for overall fatalities.

The study found that the equivalent of 2.5% of Iraq's population had been killed since fighting began more than three years ago. The study, by the Johns Hopkins University's Bloomberg School of Public Health in Baltimore, was based on household interviews - not a body count - and was a follow-up to a report by the same group two years ago.

An Iraqi government spokesman, Ali al-Dabbagh, said in a statement that the toll in the report "exceeds the reality in an unreasonable way".

The report "gives inflated numbers in a way that violates all rules of research and the precision required of research institutions", he said.