New York, New York - Dense wildfire smoke billowing down from Canada set off unhealthy air quality alerts across the US on Friday, triggering concern over the weekend's World Cup final outside New York.

Detroit and Chicago posted air quality index readings in the "hazardous" range, with tracker IQAir saying they were the top polluted cities in the world.

Winds drifting south meant the capital, Washington, was also hard-hit, coming in at the second-highest "very unhealthy" ranking on the index, when authorities urged all people to avoid unnecessary outdoor activity.

In New York and neighboring New Jersey, where the final will be played on Sunday in an open stadium, the metro area was experiencing air that could be unhealthy for sensitive groups, an improvement after smog on Thursday made the Manhattan skyline barely visible.

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But forecasters at the National Weather Service warned the smoke may thicken overnight into Saturday morning.

Tournament organizers are "monitoring closely," White House World Cup task force executive director Andrew Giuliani told a briefing.

Peter Mullinax, a meteorologist for NWS, told AFP that winds over the Great Lakes could push more smoke into the Northeast, which could keep skies hazy.

But he said forecasts for that region are expecting some improvement.

"I don't believe that this should be as impactful as if you might be playing a game today," Mullinax said.

The issue for Sunday's game, said Joel Dreessen, an air quality forecaster for the state of Maryland, is whether more smoke spills south after weekend storm systems.

"Some of the models are starting to indicate that we'll start to pull down some smoke," he told AFP.