US President Donald Trump has threatened additional tariffs on Canada as smoke from hundreds of wildfires spreads across the border, creating hazardous air conditions in several major American cities.

Trump accused Canada of failing to manage its forests and brush, describing the cross-border smoke as unhealthy air entering the United States. He said he would contact Canadian Prime Minister Mark Carney and raised the possibility of new trade measures over what he called Canadian negligence.

Canadian officials rejected the accusation and stressed that wildfire emergencies require cooperation rather than blame. Ontario Premier Doug Ford urged US lawmakers to provide firefighting assistance, noting that Canada has previously helped American communities respond to wildfires and hurricanes.

Canada reported roughly 888 active fires on Friday, with most classified as out of control. More than 190 were burning in Ontario. Nearly three million hectares have burned nationwide during the current season.

Smoke has spread through parts of the northern United States, including Minnesota, Michigan, Ohio, Pennsylvania, Washington and New York. Hazardous-air alerts prompted outdoor-event cancellations, flight disruptions and advice for residents in affected areas to remain indoors or wear protective masks.

Experts say the causes are more complex

Wildfire specialists said smoke follows atmospheric winds and does not respect national borders. Major fires in the United States have also affected Canada in previous years. Many of the current Canadian fires are burning in remote forests where early detection and containment are difficult.

Experts linked the recent surge in fires to sustained heat and below-average rainfall in northern Ontario. They also said climate change is contributing to hotter and drier conditions that allow fires to ignite and spread more easily. Some of the fires were started by lightning.

Canada said the two countries remain in regular contact under longstanding reciprocal firefighting arrangements. Federal officials said Canada has invested billions of dollars in forest sustainability and fire prevention while coordinating evacuations and emergency operations with provincial and Indigenous communities.

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Sources: BBC News and CBS Evening News. This report is an original summary of the cited reporting.