White House Press Secretary Sarah Huckabee Sanders confirmed that Trump's proposed tariffs would be formalized by week's end. She said the final plan could allow countries to be excluded.
"There are potential carve-outs for Mexico and Canada based on national security and possibly other countries as well, based on that process," Sanders said. She added that decisions would be made on a "case-by-case and country-by-country basis."
Previously administration officials had resisted any exemptions, warning it would open a Pandora's box.
Trump's tariff authority comes from a U.S. law that allows the president wide discretion to apply duties or other restrictions if the Commerce Department has found imports present a threat to national security. That determination was made -- the first time a U.S. president has done so under the law in decades.
After formally being announced by the president, the tariffs must be implemented within 15 days, Hillman said. That window would give nations and companies time to request exemptions.
Canada is by far the largest single shipper of steel and aluminum to the United States. And Mexico, South Korea and Japan, as well as North Atlantic Treaty Organization members such as Germany and Turkey, stand to be among the biggest losers if the tariffs take effect.
China, by contrast, accounted for just 2.4 percent of all iron and steel imports to the United States last year, thanks to many prior American dumping duties placed against various Chinese steel products in preceding years.
"I think ultimately that Mexico and Canada are part of the solution on steel, whether that's declared later this week or down the road," said Scott Paul, president of the Alliance for American Manufacturing, a group that has advocated for strong actions to protect U.S. steel producers.
Paul added, however, that "we must ensure that the governments of Mexico and Canada are vigilant against circumvention, join the U.S. in active trade actions to keep excess steel out of the North American market, and renegotiate NAFTA to work better for workers in all three nations."
China, for its part, has said very little about the proposed tariffs, apparently having decided to wait until the duties are finalized and made public. The Chinese know that Trump in the past has said one thing and changed his mind later.