US Defence Secretary Donald Rumsfeld has said Iraq may have destroyed its weapons of mass destruction before the US went to war against Saddam Hussein in March.
Rumsfeld said the search for banned weapons will continue
Mr Rumsfeld said the search for hidden weapons was continuing and it will "take time" to investigate hundreds of suspected sites.
The BBC's Washington correspondent Justin Webb says Mr Rumsfeld's remarks are the closest the Bush administration has yet come to an admission that it may never find any weapons of mass destruction in Iraq.
The allegation that Saddam Hussein possessed chemical, biological and nuclear weapons was the main reason why the US attacked Iraq.
The former Iraqi regime consistently insisted it had destroyed all such weapons in compliance with United Nations resolutions.
In other developments:
Might never know
- Two US soldiers are killed and nine are injured in an attack by Saddam Hussein loyalists in Fallujah, west of Baghdad
- White House spokesman Ari Fleischer calls Iran's actions against Osama Bin Laden's al-Qaeda network "insufficient"
- UK Prime Minister Tony Blair heads to Kuwait amid speculation he might visit Iraq
- The new UN special representative for Iraq, Sergio Vieira de Mello, says he will arrive in Baghdad by 2 June
- US military officials say two more senior members of the former regime have been captured - Sayf al-Din al-Mashhadani, a Baath Party chairman and militia commander in Muthanna, and Sad Abd al-Majid al-Faysal, the Baath Party regional chairman for Salah al-Din
In a speech in New York, Mr Rumsfeld said he did not know why Iraqi troops had not used chemical weapons against coalition forces.
He suggested the Iraqis might have been caught off guard by the speed of the US-led advance on Baghdad.
"It is also possible that they [the Iraqis] decided that they would destroy them [weapons of mass destruction] prior to a conflict," he said.
Mr Rumsfeld said: "We don't know what happened [to Iraq's banned weapons]. We may actually find out what happened."
He said, however, that the US military believed it had found two mobile biological weapons laboratories in northern Iraq.
The defence secretary also issued a new warning to Iran not to interfere in the reconstruction of Iraq.
Mr Rumsfeld said Iran was "being unhelpful today with respect to Iraq".
"Iran should be on notice; efforts to try to remake Iraq in Iran's image will be aggressively put down," he said.
Mr Rumsfeld's comments about Iraq came amid further accusations from Washington that Tehran has been harbouring fugitive members of Osama Bin Laden's al-Qaeda network and trying to develop nuclear weapons.
Iran has denied all such allegations.