Canada wildfires have sparked fresh tensions between the United States and Canada after President Donald Trump threatened new tariffs, blaming Canadian authorities for wildfire smoke that has spread across large parts of the northern United States.

Trump described the smoke as an unwanted intrusion and said the United States was being “invaded by filthy, polluted, and unhealthy air.” He accused Canada of failing to manage its forests effectively and said he planned to speak with Canadian Prime Minister Mark Carney to seek answers.

Canadian authorities reported about 888 active wildfires nationwide on Friday, with most burning out of control. More than 190 fires were burning in Ontario alone.

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Canada’s Emergency Management Minister Eleanor Olszewski defended her country’s wildfire response, saying Canada and the United States have worked together for decades to combat cross-border wildfires. She cited bilateral cooperation agreements dating back to 1982 and 2025 and said Canada had invested around $12 billion in forest sustainability and wildfire prevention.

Olszewski described wildfires as “a challenge that knows no borders” and stressed the importance of continued cooperation between the two countries.

Meanwhile, US lawmakers, including John James and John Moolenaar, urged stronger action in an open letter, arguing that Canada had underinvested in forest management and failed to tackle arson effectively.

Scientists, however, said weather patterns and climate conditions play a significant role in spreading wildfire smoke across international borders.

Dr Patrick James of the University of Toronto said, “Weather doesn’t care about international borders.”

Nearly 3 million hectares have burned across Canada this year, while hazardous air quality has affected cities including Detroit, Chicago and several other communities in the northern United States.

The dispute comes as relations between Washington and Ottawa remain strained over trade. Trump imposed tariffs on Canadian goods last year despite decades of largely free trade between the neighbouring countries, and both governments have yet to reach a new trade agreement.

Air quality monitoring company IQAir reported that Detroit recorded the world’s worst air quality during the latest wave of wildfire smoke.