Such tariffs would be "entirely unjustified" and "Canadians will resist strongly and firmly if necessary", he said during a conference on artificial intelligence(AI) in Paris.

"Our response will of course be firm and clear," Trudeau told AFP, after Trump said last month that the US would impose tariffs on steel from Canada, which is among the biggest steel importers to the United States.

"We are the US's closest ally," Trudeau said. "We will be working with the American administration over the coming weeks to highlight the negative impact on Americans and Canadians of these unacceptable tariffs."

Trump last month made good on a promise to slap 25 percent tariffs on steel and aluminium imports, stepping up a long-promised trade war despite warnings from Europe and China.

Trump also signaled that he would look at imposing additional tariffs on automobiles, pharmaceuticals and computer chips.