The top US weapons inspector in Iraq has hinted to Congress that "surprises" lie ahead.
David Kay was sent to Iraq by the CIA
David Kay, a special adviser to the CIA, said solid progress was being made, but would not be drawn on whether any actual weapons of mass destruction (WMD) had been found.
Mr Kay said that Iraqi scientists were revealing new secrets about work on WMD and progress was also being made to unravel the regime's concealment tactics.
Mr Kay's comments come as pressure mounts on President George W Bush and the UK Prime Minister, Tony Blair, to provide evidence of an Iraqi weapons programme to back up their decision to go to war.
The BBC's Rob Watson in Washington says there is currently no more sensitive topic in Washington than the search for Iraqi WMD.
Mr Kay told senators that most of the search work was being done at areas identified by Iraqis since the war which had not been on any US target list.
People "should not be surprised by surprises", he said.
A senior Democratic Senator warned that providing concrete proof of real WMD, not plans to develop them, was essential to justify the US-led war.
Senator Pat Roberts, the Republican chairman of the Senate Intelligence Committee, said afterwards:
"I think in view of a lot of criticism, I would not be
surprised if there is a surprise that would end up changing a lot of people's minds."
Reasons for war
However, Senator Roberts also admitted never having expected to find a "smoking missile or the smoking gun".
"I think [that] would be an incredible piece of luck," he said.
The senior Democrat on the committee, Senator John Rockefeller, said a distinction had to be made between weapons and weapons programmes.
"Programmes don't do it... it was weapons that we were told about," he said.
"I want to see this validated, in the sense that we went to war for the right reasons, and that would be weapons of mass destruction."
Our correspondent says that, given the continuing controversy over whether the threat posed by Iraq before the war was exaggerated, the Bush administration now knows any evidence will have to be convincing.
Web of deceit
Mr Kay said his team was building a "solid case that [would] stand" and that he would welcome
international scrutiny once the evidence was assembled.
A former chief weapons inspector at the UN, Mr Kay was sent to Iraq by the CIA to lead the search for weapons of mass destruction.
He stressed that the search had switched from sites identified by the coalition before the war as potential arms plants to areas picked out by Iraqis themselves.
"Almost every one of them is one that we did not know about until we were led to it by Iraqis or the documentation we have seized," he said.
Both the US and Britain accused Saddam Hussein of misleading UN inspectors before the war and Mr Kay said he was gathering evidence of deception.
"The active deception programme is truly amazing once you get inside it," he said.
"We have people who participated in deceiving UN inspectors now telling us how they did it," the US inspector added.