Former United Nations' chief weapons inspector Hans Blix has challenged U.S. President George Bush's assertion that Iran poses a nuclear threat and the world should take pre-emptive action.

Bush has recently renewed calls for a missile defence shield in Europe, issuing grim warnings that Iran could have a ballistic missile capable of reaching Europe and the U.S. by 2015.

Blix, who is the executive chair of the Weapons of Mass Destruction Commission and headed the search for WMDs in Iraq prior to the U.S. invasion, acknowledged Iran has a nuclear enrichment program and has long-range missile capability. But he told CTV's Canada AM that Bush is over-reacting.

"Right now, I don't think it's a threat," Blix said.

"President Bush talks about 2015. There's time to negotiate with Iran and to carry out those negotiations in a sensible manner. I think they use too much sticks and they should use more carrots, just as they've done in the case of North Korea where they are making some headway."

And if the U.S. does move forward with a missile defence plan, it should only be done in co-operation with other nuclear powers, including Russia and China, Blix said.

Any unilateral movement could trigger an arms race over fears that any country that perfected a missile defence shield would have an incredible advantage over the others.

"We need to wake up to that," Blix said. "Al Gore has woken us up to the danger of global warming as one inconvenient truth, but there is another one and that is we are moving into arms races again."

The U.S. plan involves placing missile interceptors in Poland and a radar system in the Czech Republic as protection against threats from "rogue" states such as North Korea and Iran.

Russia is against the plan, fearing it would constitute a threat against its national security.

Blix was in Toronto as part of United Nations Disarmament Week, speaking about what the world would look like without weapons of mass destruction.

Though that reality is probably a long way off, Blix said it is possible, with the help of the United Nations.

"It's a long perspective and I'm not saying we can eliminate weapons by tomorrow. It will take decades. But I think we'll get rid of them because the interdependence is so great in the world today and we'll need the UN for this process. There is no alternative. The UN has some weaknesses but it is something we must improve on. It's a mechanism. It's something that can be used."

Progress is being made, Blix said. The number of nuclear weapons thought to be in the world has gone from a peak of 55,000 warheads during the Cold War, down to a current 27,000, and the number will continue to fall.

But he said there also needs to be a concerted effort to tackle the issue in the earlier stages.

"We need to go on also to stop the production of similar materials for bombs -- plutonium and enriched uranium -- a gradual process of disarmament."