The state secretary's comments on Thursday came after a prominent US think tank reported it did not expect any large stockpiles of biological and chemical weapons to be found.
But US officials have said that stockpiles could be hidden in relatively small areas, and have criticised the Carnegie Endowment for International Peace.
And Powell stressed that the information he gave was exactly the same as that presented to Congress and the public, adding "this game is still unfolding".
However, analysts see signs of fading expectations of finding any chemical or biological weapons in Iraq - such as the possibility that CIA adviser David Kay, in charge of the hunt for banned weapons in Iraq, may leave his job.
Former CIA Director Stansfield Turner said he was in general agreement with the Carnegie think tank presentation and said he believed no Iraqi weapons of mass destruction had been found for several reasons.
It was "a combination of the intelligence people overestimating what was there and policy people exaggerating the intelligence estimates".
Turner added that the fact that the inspections and destruction of weapons by the UN from 1991 to 1998 eliminated a lot and made it very difficult for the Iraqis to start it up again because they couldn't get the materials or the equipment.
The failure to find Iraqi weapons of mass destruction looks set to dog the Bush administration in an election year amid persistent accusations it exaggerated evidence in making a case for war.
Possibilities become facts
The Carnegie Endowment report compared public and declassified intelligence information with statements made by administration officials.
It concluded that the administration made the threat from Iraq sound more dire than the underlying information.
It was "a combination of the intelligence people overestimating what was there and policy people exaggerating the intelligence estimates"
Former CIA Director
"We have found and have gone to some length to define and lay out serious misrepresentation of the facts over and above what was in the intelligence findings," Jessica Mathews, president of the think tank and one of the authors, said.
In one example, she said UN weapons inspectors said the amount of biological growth medium Iraq had could produce three times as much anthrax as it had declared if it used all that growth medium to produce anthrax.
Later, in an October 2002 speech in Ohio, President George Bush said: "The inspectors … concluded that Iraq had likely produced two to four times that amount.
"This is a massive stockpile of biological weapons that has never been accounted for and is capable of killing millions."
Mathews said this was an example of how a possibility cited by the inspectors became a likelihood and then a stockpile in Bush's speech.
"And finally, biological agent is transformed into weapons" which would require highly sophisticated delivery systems if they were capable of killing millions, she said.
Joseph Cirincione, director of the non-proliferation project at Carnegie and an author of the report, said administration officials dropped caveats.
"In that process they changed something that is an opinion into a fact, and they consistently did this," he said at a briefing on the report.
"The problem is it gives a misleading impression to the public, to the US Congress, about what you know and how certain you are about that knowledge," Cirincione said.