The CIA official in charge of the search for weapons of mass destruction (WMD) in Iraq has said no such weapons have so far been found.
Evidence of weapons programmes was discovered
However, the interim report says significant amounts of equipment and weapons-related activities have been discovered, all of which were concealed from United Nations inspectors.
The report's author, David Kay, said it is too early to reach definitive conclusions and much work remains to be done.
The failure of the US-led coalition to make any significant discoveries has led to criticism of the decision to invade Iraq but the report does not settle the debate between critics and supporters.
Mr Kay, who heads to Iraq Survey Group (ISG), said: "We have not yet found stocks of weapons, but we are not yet at the point where we can say definitively either that such weapon stocks do not exist or that they existed before the war."
He said much evidence about Iraq's banned weapons programmes which once existed had been "irretrievably lost".
IRAQ SURVEY GROUP
Set up in May
Kay appointed by CIA director George Tenet
1,200 experts from the US, Britain and Australia
Headquarters in Washington, offices in Baghdad and Qatar
The ISG, however, found significant evidence of continuing Iraqi weapons research and development, he said.
Teams have discovered clandestine laboratories and found live botulinum toxin - which could be used to make biological weapons - at an Iraqi scientist's home, according to the report.
It says plans were discovered for missiles capable of flying up to 1,000 kilometres (625 miles) - well beyond the 150 km range limit (93 miles) set by the United Nations.
There were also alleged contacts with North Korea to obtain missile technology.
Mr Kay said additional information was beginning to corroborate reports of human testing activities using chemical and biological substances.
BBC defence correspondent Jonathan Marcus says that while the report contains no smoking gun, its information, if correct, suggests that Iraq had embarked upon a systematic effort to keep its weapons programme going.
Speaking to reporters after briefing Congress behind closed doors, Mr Kay said investigators had found evidence to suggest Saddam Hussein was trying to revive a nuclear weapons programme "at the most rudimentary level".
In his report, Mr Kay said Iraqi scientists and former government officials had told investigators Saddam Hussein nevertheless "remained firmly committed to acquiring nuclear weapons".
The report says attempts to conceal Iraq's WMD programme continued even after the US-led war which toppled Saddam Hussein.
It has prompted mixed reactions in the US and beyond.
Hans Blix, who led the team of UN weapons inspectors in Iraq before the conflict, told the BBC that America still had not come up with any evidence that Iraq had posed a great enough threat to justify war.
"I don't think there are any surprises. The most important point is that they confirm that they have not found any stocks of weapons of mass destruction of any kind," he said.
US Senator Jay Rockefeller said it was extraordinary that a decision was made to go to war when so little evidence was available.
"You just don't make decisions like we do and put our nation's youth at risk based upon something that appears not to have existed," he said.
But UK Foreign Secretary Jack Straw said the contents of the report justified the removal of the Iraqi leader.
In Australia, Prime Minister John Howard said he had no regrets over his country's role in the US-led war on Iraq.
Mr Howard on Friday stressed the interim nature of the report, which he said "has already demonstrated a great deal of concealment by the former (Iraqi) regime and a clear intention to develop weapons programmes".
Mr Kay's report comes as Congress debates President George W Bush's request for $87bn for spending on Iraq and Afghanistan.
One element of that budget is $600m to pay for further searches for evidence that Saddam Hussein did possess weapons of mass destruction,.