Heavily populated southern Canadian cities could be littered with the debris of nuclear warheads if missiles fired at the U.S. from North Korea were shot down by a missile-defence shield over Canada, says a prominent U.S. physicist.

Ted Postol, a physicist at the Massachusetts Institute of Technology, told The Winnipeg Free Press that even if a defensive strike were launched immediately, there would be only about five minutes to pinpoint and demolish an enemy missile.

Missiles fired at the U.S. from North Korea would pass over Canada.

Postol said a destroyed warhead would disperse the radioactive material over a large area.

But University of Manitoba political scientist George Maclean told the Free Press that most of the debris would likely burn up in the atmosphere and the rest would disseminate over the northern hemisphere.

Critics of the missile defence plan read an open letter to Prime Minister Paul Martin on Monday, warning him "even future, expanded versions of the system would be vulnerable to enemy countermeasures and therefore unable to provide a real defense."

"Our analysis clearly shows that the missile defense system currently being fielded will not provide protection against long-range ballistic missile attacks," says the letter from David C. Wright and Jonathan Dean, both from the Union of Concerned Scientists.

"It seems to me that the trend in missile defence is inevitably into the space component and one will lead to another," Dean said at a Monday news conference.

Dean said a missile-defence system was not the best defence, particularly because the weapons used "have uncertain effects and they are very possibly negative effects."

Dean added, "The only plausible threat to North America is from North Korea, which has voluntarily disbanded its missile tests."

Dean is the former US representative and deputy representative to the NATO-Warsaw Pact force reduction negotiations in Vienna.

Wright is the co-director and senior scientist of the Global Security Program at the Union of Concerned Scientists.