Rather than forcing Canada and Mexico into concessions on Nafta, analysts say, the tariff move is likely to strengthen Canada's and Mexico's resolve to not back down on unconventional U.S. demands for Nafta, such as the elimination of international panels to resolve trade disputes and a new clause under which the pact would expire if not explicitly renewed every five years.

The U.S. "may see this as a cost-free way of applying additional pressure. But the result will be the opposite," said Gordon Ritchie, a former Canadian ambassador for trade negotiations.

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