"I did not think so at first. But the US is incredibly dependent on oil," Swedish news agency TT quoted Mr Blix as saying at a security seminar in Stockholm.
"They wanted to secure oil in case competition on the world market becomes too hard."
Mr Blix, who has become a vocal critic of US leaders since he retired from the UN last year, reportedly said another reason for the invasion was a need to move US troops from Saudi Arabia.
Competition over oil is creating tension between the US and China, Mr Blix said, suggesting nuclear power as a more environmentally friendly source of energy.
He defended the UN, despite recent scandals including allegations of corruption in the oil-for-food program for Iraq.
"The criticism is, in my view, a revenge from American political circles for the defeat over Iraq," Mr Blix was quoted as saying.
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