Singapore has topped the world's most-polluted-cities ranking as thick haze from Indonesian wildfires chokes the city-state during its Formula One weekend.

Air quality monitors recorded an index reading of 216 on Friday mid-morning, in the "very unhealthy" range, according to Swiss monitor IQAir. Singapore led the global list this week, with Kuala Lumpur second. The haze has reduced landmarks like Marina Bay Sands to faint silhouettes.

The smoke is drifting from wildfires in Sumatra and Kalimantan, where peatlands are cleared for plantations. An expected "super" El Nino has made this year's fire season unusually intense.

Singapore's National Environment Agency reported a 24-hour pollutant index of 105 to 140 on Wednesday evening, already in the unhealthy band.

The timing is awkward for the city, which is hosting the glitzy F1 night race alongside international conferences. Organisers are monitoring conditions, though the race has continued through haze episodes before.

Residents have been advised to limit strenuous outdoor activity until the winds shift and the air clears.