Blix, a Swedish diplomat, said on Wednesday inducements may be good enough to draw Iran away from an enrichment programme that he said could accelerate by about two years the weapons' production.
Speaking at the 25th anniversary of the Arms Control Association, a private group, Blix criticised the Bush administration as putting arms control in reverse in many ways.
Among the policies singled out by the former head of the International Atomic Energy Agency were US opposition to a test bank treaty, a proposed missile shield and lack of interest in establishing nuclear free zones around the world.
"What is the situation now?" Blix asked. "Not a rosy one."
Like the administration, the former UN official said he favoured trying to induce Iran to halt programmes that could produce nuclear weapons.
But he disagreed with trying to bring Iran before the UN Security Council with the possibility of economic and political sanctions if Iran did not agree to reopen negotiations with the European Union.
"I think that would harden Iran's attitude," Blix said. "It does not help very much to go to the council."
He suggested the UN agency as an alternative forum for discussion, but said what was important was offering Iran a package of "carrots" that would include a US pledge not to attack Iran as well as trade and other economic incentives, some of which the Europeans offered.
The United States has offered North Korea a written promise not to attack it, and a similar offer should be extended to Iran, he said.
Before the US-led war in Iraq, Blix oversaw more than 700 inspections in search of weapons of mass destruction that came up empty. Had the US attack not stopped UN searches, Blix said in a few more months' the UN agency could have completed its job.
George Bush, the US president, and his senior advisers rationalised the war on Iraq, which deposed Saddam Hussein, the Iraqi president, by saying Iraq had secret caches of unconventional weapons.
Later, the administration blamed faulty intelligence for its unproven claims.