MONTREAL – Canadian Prime Minister Justin Trudeau promised aluminium and steelworkers on Monday he would defend them against possible US tariffs and called US President Donald Trump to stress that "mutually beneficial" cross-border supply chains should be preserved.

Trump said last week he would impose import tariffs of 25% on steel and 10% for aluminium, effective later this month, but exempted Canada after an intense lobbying campaign arranged by Ottawa.

"We are ready to take action whenever action is required ... we had your backs last week and we always will," Trudeau said after visiting a Rio Tinto smelter in Alma, Quebec, his first stop on a tour this week of Canada's steel and aluminium regions.

In a phone call from the smelter town, Trudeau thanked Trump for the "special consideration" extended to Canada on the tariffs, and emphasised the importance of preserving "mutually beneficial" supply chains to support jobs and businesses on both sides of the border, his office said.

It was the second call in a week that Trudeau made to Trump on the tariffs issue.

Canada, the biggest supplier of steel and aluminium to the US, escaped Trump's import duties along with Mexico, but the two countries could still face duties if they fail to reach a deal with the Trump administration on modernizing the North American Free Trade Agreement.

Trudeau said Canada had a lot more work to do and would press Washington to ensure the exemptions were permanent. Canada has said that tariffs would hurt both the US and Canada.

The Canadian government has vowed to retaliate if duties are imposed, but the prime minister did not answer directly when asked what measures it might take.

Trudeau rejected calls by some Quebec union leaders to take a harder line in NAFTA talks and leave the table if Canada cannot get a better deal.

"We are negotiating the NAFTA accord in good faith and we will continue to do so, but I don't want the president to think he can bring tariffs on Canadian steel and aluminium without there being consequences," he told Radio-Canada earlier in the day.

Later this week, Trudeau is due to visit the Ontario steel city of Hamilton, where workers are on edge.