President Donald Trump is threatening to increase tariffs on Canada over wildfire smoke that has blanketed large swaths of the Midwest and Mid-Atlantic, joining several Republicans who have called for the U.S. ally to be punished for the intense air pollution.

“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!” Trump wrote on Truth Social on Friday, adding: “This is Willful Negligence, and becoming a yearly occurrence, costing the United States Billions of Dollars, which cost of this pollution must of necessity be added to the TARIFFS Canada is currently paying.”

Trump did not elaborate on his tariffs threat.

Smoke from hundreds of Canadian wildfires has caused air quality from Detroit to Washington, D.C., to plummet to unhealthy levels in recent days.

There are dozens of active wildfires in the U.S. as well. A Canadian helicopter pilot was was killed last week in a crash while fighting a fire in Colorado.

Trump is not the only Republican who has criticized Canada over the wildfire smoke. Earlier this week, four House Republicans from Michigan wrote a letter to Canadian Prime Minister Mark Carney with a warning that appeared to allude to Trump’s threat to annex Canada.

“Sovereignty comes with responsibility,” the lawmakers wrote.

“This is the third consecutive year we have had to write to Canadian officials about a crisis that Canada has the tools to prevent and has chosen not to,” they wrote, later adding: “If Canada will not manage its forests to prevent these fires, the United States will look elsewhere, and act on our own, to protect our people.”

Sen. Bernie Moreno, R-Ohio, has also said he intends to introduce a bill “to sanction Canada and the responsible Canadian government officials for this atrocity.”

In a statement, Moreno’s office said: “Canada’s government failed to invest in wildfire prevention methods including forest thinning, fuel reduction, prescribed burns, and stronger enforcement against arson.”

Hotter temperatures and drier conditions as a result of the climate crisis have been major drivers of recent wildfires in North America. The Trump administration has cut funding for climate science, withdrawn the U.S. from global bodies and agreements aimed at tackling climate change and promoted the fossil fuel industry while rolling back renewable energy initiatives.

In response to the GOP complaints, some Canadian officials have noted that their country has helped with firefighting support in the U.S. during recent wildfires.

“If there’s some politicians out there chirping away, well, maybe what you should do rather than complain is send support, send help,” Ontario Premier Doug Ford said on Friday, “because we have done the exact same thing for our American friends.”

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