President Bush's national security adviser has acknowledged there may have been flaws in intelligence about Iraq but rejected calls for an independent investigation.
"I think that what we have is evidence that there are differences between what we knew going in and what we found on the ground," Ms Condoleezza Rice told CBS.
"That's not surprising in a country that was as closed and secretive as Iraq, a country that was doing everything that it could to deceive the United Nations, to deceive the world," she added.
President Bush based his decision to invade Iraq last year on what he called a "grave and gathering danger" posed by Iraq's weapons. He acted without UN backing, and refused to allow weapons inspectors complete their attempts to discover Saddam Hussein's alleged weapons of mass destruction.
But Ms Rice defended President Bush's decision and said the United States may never learn the whole truth about Iraq's arms capabilities because of looting.
After the top US weapons inspector concluded Iraq had no stockpiles of biological or chemical weapons, the White House said on Monday it would review pre-war intelligence.
The weapons issue is a major topic in campaigning for the November presidential election, with Democrats saying Mr Bush misled the country over the level of the Iraqi threat.