Speaking on Swedish Radio, Blix questioned whether Saddam Hussein posed an immediate threat to his neighbors or to the United States, and said the Bush administration had other reasons to invade besides destroying alleged weapons of mass destruction.
He said the U.S. also needed to show what he calls "striking power" following the Sept. 11 terrorist attacks.
The comments were not Blix's first criticisms of the war, which the United States and Britain said was launched because intelligence indicated Iraq possessed illegal biological and chemical weapons material and was working towards developing nuclear arms.
Since the fall of Baghdad on April 9, no illegal weapons have been reported found, despite extensive searches and interviews with numerous Iraqi officials and scientists.
Lawmakers in Washington and London have questioned their respective governments' use of prewar intelligence.
Some U.S. analysts have claimed in published reports that they felt pressure to skew intelligence to fit the Bush administration's case. Declassified documents have indicated significant doubt and disagreement within the intelligence community over the threat posed by Iraq.
The White House admitted last month that a line in the State of the Union referring to alleged Iraqi attempt to buy uranium in Africa should have been omitted because the evidence was questionable.
Committees in the House and Senate are reviewing the intelligence used to justify the war.
Administration officials, who had downplayed the importance of weapons a few weeks ago, have recently expressed guarded optimism that evidence will surface pointing to weapons programs.
On Tuesday, Defense Secretary Donald H. Rumsfeld cited the recent discovery of buried airplanes in Iraq and suggested evidence of weapons also would be found.
U.S. officials reported last week that search teams, some hunting for alleged weapons of mass destruction, found dozens of fighter jets from Iraq's air force buried beneath the sand.
"So until you find somebody who tells you where to look or until nature clears some sand away and exposes something over time, we're simply not going to know," Rumsfeld said. "But as we all know, the absence of evidence is not evidence of absence."
CIA adviser David Kay, who is serving as a special adviser for the weapons search, told lawmakers last Thursday that inspectors have found physical evidence of Iraqi activity on weapons of mass destruction.
Without offering any detail, he said investigators had made a "tactical and strategic decision" to focus on biological rather than on chemical or nuclear programs.
Rumsfeld indicated that Kay's evidence was being checked for possible presentation at a later date. Democrats say that evidence might not satisfy them.
"Signs of a weapons program are very different than the stockpiles of biological and chemical weapons that were a certainty before the war," said Sen. Jay Rockefeller, D-W. Va. "We did not go to war to disrupt Saddam's weapons program, we went to disarm him."
And The Washington Post reports Iraqi scientists interviewed by the U.S. deny that Saddam had restarted his nuclear weapons program or secretly developed chemical and biological weapons since U.N. weapons inspectors left the nation in 1998.
The newspaper said interviews with four senior scientists and more than a dozen lower-level scientists have failed to yield any evidence to support the U.S. contention that Saddam was developing weapons of mass destruction.
The best known of these researchers, nuclear scientist Mahdi Obeidi, dug up parts and plans for a gas centrifuge that he buried in 1991 following the first Gulf War.
But Obeidi also told U.S. investigators that Saddam's nuclear program had been dormant for years and that aluminum tubes purchased by Iraq were for use on rockets and not the development of a nuclear weapon, as the U.S. claimed.
Across the Atlantic, two parliamentary committees are probing Prime Minister Tony Blair's case for war. One has already scolded Downing Street for a February dossier on Iraq's alleged arsenal that plagiarized a dated student thesis.
One weapons expert who testified before that committee, David Kelly, committed suicide last month and was buried Wednesday. He killed himself after he was named as the likely source for a BBC report suggesting Blair aides had over hyped a particular piece of prewar intelligence.
Downing Street denied that specific charge. One parliamentary committee largely refuted it.
After his death, the BBC confirmed Kelly had been the source.
Blair appointed a senior appeals judge, Lord Hutton, to inquire into the circumstances of Kelly's death.
One of Blair's spokesmen apologized Tuesday for suggesting that the dead man may have been a "Walter Mitty" fantasist.