The US defence secretary has said that finding any weapons of mass destruction in Iraq is still a major concern.
In a presentation before the war, the US tried to prove Iraq still had banned weapons
Asked at a news conference whether the rationale behind the war required that such banned weapons be found, Donald Rumsfeld said he did not "quite get the thrust of the question", but agreed that "it obviously is important to find them".
Washington based its case for an invasion on Iraq's alleged possession of nuclear, chemical and biological weapons, but UN inspectors failed to find definitive examples of such possession and fears that Iraqi forces would use such weapons have failed to materialise.
Earlier, UK Prime Minister Tony Blair said he had no doubt they would be found.
Mr Rumsfeld said weapons production facilities needed to be "found and secured".
He warned of a "nexus between terrorist states... and terrorist groups".
The possibility that "some of these weapons could leave the country and [get into] the hands of terrorist networks would be a very unhappy prospect," he said.
Facing questions in parliament, Mr Blair said it was not surprising that no banned weapons had yet been found, only three weeks after US and British forces invaded Iraq to overthrow Saddam Hussein.
But he added: "I have no doubt at all that these weapons of mass destruction exist... The truth is there has been a six-month campaign of concealment."
It was important for the world to have any banned weapons finds "objectively verified", perhaps by United Nations inspectors, he added.
This concern was echoed by Mohamed ElBaradei, head of the International Atomic Energy Agency (IAEA).
Mr ElBaradei said any suspected weapons finds by the UK or the US should be verified by UN inspectors "to generate the required credibility".
Mr Blair and Mr ElBaradei's stress on external verification of suspect weaponry found is being seen as tacit acknowledgement that some observers believe coalition forces could be tempted to fabricate such "evidence" if it proves elusive.
In an interview with Spanish daily El Pais, chief UN weapons inspector Hans Blix hinted he believed Iraq's contended possession of weapons of mass destruction had served as a pretext for a US-led invasion.
"There is evidence this war was planned well in advance," he said.
"You ask yourself a lot of questions when you see the things they [the US] did to try and demonstrate that the Iraqis had nuclear weapons, like the fake contract with Niger," he said.
He was referring to the discovery by UN inspectors that documents the US alleged proved Iraq had tried to buy uranium from the African state had been forged.
Mr Blix said he thought finding banned weapons in Iraq was now a low priority for coalition forces - and that "today, the main aim is to change the dictatorial regime of Saddam Hussein".
US commanders continue to stress the threat posed by banned weapons.
On Wednesday, US Brigadier General Vincent Brooks warned that Saddam loyalists were still holding out in some parts of northern Iraq, and might resort to using weapons of mass destruction.
On Monday, US defence officials said that initial field tests on a number of chemicals found near the city of Karbala suggested the possible presence of the nerve agents sarin and tabun, as well as mustard gas.
But there have already been a series of false alarms.
On Saturday, US officials admitted to the latest, when they said tests on thousands of vials of white powder touted to be a possible chemical warfare agent revealed it was more likely to be an explosive or antidote.