Wildfires burning in Canada have sent the noxious smog billowing across the border, with residents in New York, Washington and the US Midwest urged to stay indoors due to dangerously low air quality.

At one point on Saturday, the smoke made New York City the most polluted city in the world, followed closely by Toronto and Washington, according to air tracker IQAir.

The marquee World Cup match, pitting Argentina against Spain, will be played Sunday in an open stadium in New Jersey, right across the river from New York City, where the city skyline was obscured by dense smoke and people wore masks outdoors.

Tournament organisers are “monitoring closely,” White House World Cup task force executive director Andrew Giuliani told a briefing.

But a respite of sorts was expected to come in the form of heavy storms hitting the Big Apple on Saturday, which could wash some of the smoke away — but also trigger flash flooding and dangerous winds.

New York Mayor Zohran Mamdani asked residents to stay indoors and avoid travel.

“Thunderstorms are expected to bring damaging winds strong enough to down trees and power lines, along with heavy rainfall that could cause flash flooding,” Mamdani wrote on X.

“Smoke will continue to affect the area through this afternoon,” the US National Weather Service said.

In the US capital, the department of homeland security and emergency management said air quality was “unhealthy for at-risk groups” including children and the elderly, urging people to limit time outdoors.

More than 950 wildfires were raging across Canada as of Saturday morning, the Canadian Wildland Fire Information System reported.

US President Donald Trump has blamed Canada for the smoke pollution, threatening to impose additional tariffs, accusing the northern neighbor of “willful negligence” by “not properly maintaining” its forests.

Advocates have stressed the connection between repeated episodes of wildfire smoke and climate change.