US President Donald Trump has threatened fresh tariffs against Canada, accusing it of "willful negligence" over wildfires whose smoke has blanketed much of the northern United States. The threat follows complaints from Republican lawmakers and comes despite Ontario's premier Doug Ford urging Washington to send firefighting help rather than criticism, highlighting fresh strain in an already tense US-Canada relationship that has been dominated by unresolved trade disputes over the past year.

As of the weekend, around 955 wildfires were burning across Canada, most out of control, with over 190 in Ontario alone and nearly three million hectares destroyed so far. The resulting smoke triggered "hazardous" air quality alerts across US states including Michigan, Minnesota, Pennsylvania, Ohio and New York, with Detroit ranked the world's worst city for air quality on Friday. Canadian officials responded by pointing to roughly C$12bn invested in forest sustainability and fire prevention, and to long-standing bilateral firefighting agreements, while some Republicans revived talk of Canada becoming the 51st US state.