American aluminum giant Alcoa Corp. AA-N is warning that a looming trade war is placing 100,000 U.S. jobs at risk in the industry as the company urges the Trump administration to exempt the metal from threatened tariffs on shipments from Canada.

Two weeks ago, U.S. President Donald Trump announced 25-per-cent tariffs on steel and aluminum, including imports from Canada, that would start on March 12.

“We’re clearly advocating based on the fact that this is bad for the aluminum industry in the U.S.,” Alcoa chief executive officer William Oplinger said on Tuesday during BMO Capital Markets’ global metals, mining and critical minerals conference in Florida.

“It’s bad for American workers. We’re advocating with the administration to, at a minimum, get a Canadian exemption, which will allow two-thirds of that metal that gets consumed in the U.S. to continue to come across the border without a tariff.”

Pittsburgh-based Alcoa’s global network for aluminum production includes operations in Quebec, the U.S., Australia and Brazil.

Mr. Trump has also said he wants to impose across-the-board 25-per-cent tariffs on goods that the U.S. imports from Canada and Mexico, scheduled to start on March 4. For Canadian energy resources shipped into the U.S., plans call for tariffs of 10 per cent for commodities such as crude oil, natural gas and critical minerals.

Canada and the U.S. both classify aluminum as a critical mineral, so it could be subject to the separate tariff at 10 per cent.

Mr. Oplinger said Alcoa is concerned about the impact of the potential stacking effect, in which the 25-per-cent tariffs against aluminum from Canada would be combined with the sweeping tariffs.

“Our view is that currently, those two tariffs would stack for a 35-per-cent net tariff coming from Canada,” he said during the conference, which was webcast from Hollywood, Fla.

“I don’t have updated numbers for a 35-per-cent tariff, but we have a view that a 25-per-cent tariff will destroy about 20,000 direct U.S. aluminum industry jobs and could result in 80,000 indirect jobs being eliminated in the U.S.,” Mr. Oplinger said.

The aluminum industry has estimated that there are nearly 165,000 workers employed directly in the U.S. sector and more than 470,000 people indirectly.

Mr. Oplinger said Alcoa faces challenges in what would be a complex decision to increase smelter capacity through major investments within the United States as a way of avoiding tariffs.

“One of the issues around the uncertainty of the tariffs is it’s very hard to make an investment decision even on something like a restart without knowing how long the tariffs will last,” he said, adding that finding low-cost energy would be crucial.

Canada is the leading supplier of both steel and aluminum to the U.S., and Canadian producers rely heavily on the U.S. market

The Canadian aluminum industry is concentrated in Quebec, including plants operated by Anglo-Australian miner Rio Tinto PLC, which also runs a smelter in Kitimat, B.C.

Canada’s aluminum sector estimates that 9,500 workers are employed to produce the metal in this country. “That is processed, transformed and fabricated into parts, components and everyday products by more than 500,000 well-paid American manufacturing workers,” the Aluminium Association of Canada said in a statement earlier this month.

The association represents Alcoa, Rio Tinto and Aluminerie Alouette.

Jakob Stausholm, CEO of London-based Rio Tinto, visited Washington recently to gain a better understanding of tariffs.

Last week, Mr. Trump proposed to introduce tariffs globally on softwood lumber in April, broadening the scope of threatened levies beyond Canada to also hit Europe.

Similar to the aluminum industry, the Canadian lumber sector is worried about the layering effect of tariffs.

Analysts say U.S. duty rates imposed on Canadian softwood could double later this year and reach nearly 30 per cent. If the threatened new tariffs are applied, that would mean a rate of tariffs and lumber duties eventually totalling 55 per cent.