U.S. President Donald Trump is threatening new tariffs on Canada over wildfire smoke that, he said, has “invaded“ American cities in recent days, calling it “Willful Negligence” on the part of Canadian authorities.

Mr. Trump, in a post to his Truth Social site on Friday, said the smoke has created an “incalculable” expense for the U.S., and the “cost of this pollution must of necessity be added to the TARIFFS Canada is currently paying.”

The U.S. President said he would call Prime Minister Mark Carney on Friday to discuss the issue, and made no concrete proposal for what tariffs, if any, he could apply. The U.S. Supreme Court, in a ruling earlier this year, constrained Mr. Trump from summarily imposing tariffs on an emergency basis.

“We are holding Canada responsible for the fact that they are not properly maintaining their Forests, and Brush therein, and the United States is being unnecessarily invaded by filthy, polluted, and unhealthy air, the quality of which is dangerous, and totally unacceptable!” Mr. Trump wrote.