UPDATE 1-Canada's Trudeau tells metal workers 'we have your backs'

(Releads with comments by Canadian Prime Minister)

MONTREAL, March 12 (Reuters) - Canadian Prime Minister Justin Trudeau on Monday sought to reassure steel and aluminum workers worried about possible U.S tariffs, telling them the government would always have their backs.

U.S. President Donald Trump last week said he would impose import tariffs of 25 percent on steel and 10 percent for aluminum, 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.

He spoke after a visit to a Rio Tinto Ltd facility in Quebec as he kicked off a tour this week of the nation's steel and aluminum regions.

Canada, the biggest supplier of steel and aluminum to the United States, 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 modernizing the North American Free Trade Agreement.

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

"The exemptions aren't a magical favor that was being done," Trudeau later said in an interview with U.S. television's CNBC.

"We made the ... true point that the integration of our steel and aluminum market is so intense across the Canada-U.S. border that millions of jobs on both sides of the border depend on continued smooth flow of trade," he said.

Canada has vowed to retaliate if duties are imposed but Trudeau 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.

"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 aluminum without there being consequences," he told Radio-Canada earlier in the day.

U.S. tariffs on Canadian steel and aluminum would be "very harmful" for industry and workers in both countries, he added. Later this week Trudeau is due to visit the steel town of Hamilton, where workers are on edge. (Additional reporting by David Ljunggren and Leah Schnurr in Ottawa Editing by Denny Thomas and Alistair Bell)