'Willful negligence': Trump threatens Canada with tariffs as wildfires worsen air quality in US states

US President Donald Trump has threatened to impose new tariffs on Canada after hundreds of wildfires blanketed large parts of the northern United States in thick smoke.

The threat comes after some US lawmakers criticised Canada over the fires, while Ontario Premier Doug Ford urged Washington to send assistance to help battle the blazes instead of complaining.

"The United States is being unnecessarily invaded by filthy, polluted, and unhealthy air," Trump wrote on Truth Social, adding that he was considering new levies over Canada's "willful negligence".

As of Friday, Canada had about 888 active wildfires, according to the Canadian Wildland Fire Information System, with most burning out of control.

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Ontario alone is battling more than 190 active fires, several of which remain uncontrolled.

Trump said he would call Canadian Prime Minister Mark Carney to demand an explanation for what he described as Canada's "willful negligence", accusing the country of "not properly maintaining" its forests and brush.

The wildfire crisis has also prompted some fellow Republicans to revive Trump's long-running suggestion that Canada should become the 51st US state, a proposal that has angered many Canadians and led some to boycott travel to the United States.

Others online have called for delaying the opening of the Gordie Howe International Bridge, a Canada-funded project linking Ontario with Michigan.

Canada has not yet responded to Trump's tariff threat. However, Carney had earlier said that tackling climate change is a shared responsibility for both countries.

Canada wildfires

The Manhattan skyline was obscured by thick haze and Chicago closed its beaches Thursday as out-of-control Canadian wildfires raged, sending smoke spewing into the United States and exposing millions of people to dangerously unhealthy air.

US states near the Canadian border, including Minnesota, Wisconsin, Michigan and Illinois, were particularly choked, while the Northeast, including New York, was also experiencing deteriorating air quality.

On Thursday evening, tracker IQAir said Chicago and Detroit were the most polluted cities in the world, with New York not far behind.

The most populous US city was shrouded by the smoke that state authorities warned contained fine particulate matter deemed unhealthy for everyone across the New York metro area and Long Island.

With inputs from agencies