"There is evidence that this war was planned well in advance. Sometimes this raises doubts about their attitude to the (weapons) inspections," Blix told Spanish daily El Pais.
"I now believe that finding weapons of mass destruction has been relegated, I would say, to fourth place, which is why the United States and Britain are now waging war on Iraq. Today the main aim is to change the dictatorial regime of Saddam Hussein (news - web sites)," he said, according to the Spanish text of the interview.
Blix said US President George W. Bush (news - web sites) had told him in October 2002 that he backed the UN's work to verify US and British claims that Baghdad was developing biological, chemical and nuclear weapons.
But he said he knew at the time "there were people within the Bush administration who were sceptical and who were working on engineering regime change". By the start of March the hawks in both Washington and London were getting impatient, he added.
Blix said that he thought the US might initially have believed Iraq possessed weapons of mass destruction -- although its "fabrication" of evidence raised doubts about even that -- but that Washington was now less convinced by its own claims.
"I think the Americans started the war thinking there were some. I think they now believe less in that possibility. But I don't know -- you ask yourself a lot of questions when you see the things they did to try and demonstrate that the Iraqis had nuclear weapons, like the fake contract with Niger," he explained.
That was a reference to US allegations -- later denied -- that Iraq had sought to purchase uranium from the west African state of Niger.
"I'm very curious to see if they do find any (weapons)," he said.
Blix said the war, which on Wednesday entered its 21st day, was "a very high price to pay in terms of human lives and the destruction of a country" when the threat of weapons proliferation could have been contained by UN inspections.
By attacking Iraq, Washington had sent the wrong message -- that if a country did not possess biological, chemical or nuclear weapons, it risked being attacked.
"The United States maintains that the war on Iraq is designed to send a signal to other countries to keep away from weapons of mass destruction. But people are getting a different message. Take the announcement North Korea (news - web sites) has just made. It's tantamount to saying 'if you let in the inspectors, like Iraq did, you get attacked'.
North Korea accused the United States on Sunday of using a UN Security Council discussion of its nuclear programme as a "prelude to war" and warned that it would fully mobilise and strengthen its forces.
"It's an important problem," Blix continued. "If a country perceives that its security is guaranteed, it won't need to consider weapons of mass destruction. This security guarantee is the first line of defence against the proliferation of weapons of mass destruction."
The 74-year-old Swede announced in March that he would step down from his post when his contract runs out in June.
Blix's reputation for independence and resisting political pressure was sorely tested as the Iraq crisis unfolded and US officials became exasperated with his measured reports on Iraqi cooperation with his inspection teams.