WASHINGTON—President Donald Trump plans to apply his steel and aluminum tariffs globally and won’t exempt allies such as Canada and Europe, a senior White House official said Friday, an approach that is likely to intensify protests over the move.

The statement that there would be no exceptions to the duties came as Trump aides started to flesh out the president’s broad Thursday announcement, in which he said the U.S. plans to impose tariffs of 25% on steel and 10% on aluminum. Officials are scrambling to finalize many details ahead of a planned rollout of the full policies next week.

“The president made clear these would be across-the-board tariffs with no exclusions,” the White House official told reporters. “One problem with exclusions is that it’s a slippery slope. Where do you stop?”

Fears of such a broad-based approach drew complaints and counter-threats from the European Commission—which said Friday it had already crafted a detailed retaliation package—and from Canadian Prime Minister Justin Trudeau, who warned of “significant and serious” disruptions to the North American economy.

The White House official said he couldn’t provide other crucial details of the still-emerging policy, such as whether the duties would apply to all steel products, or would exempt semifinished products, as has been the case when broad steel-import limits have been imposed in the past.