Singapore's air quality index reached unhealthy levels with conditions deteriorating through the day, marking the worst return of the haze to the city since vast parts of Southeast Asia were affected in 2015.
Last year's haze outbreak was among the worst in memory, shrouding Malaysia, Singapore, and parts of Thailand in acrid smoke.
The blazes are started illegally to clear land, typically for palm oil and pulpwood plantations, and Indonesia has faced intense criticism from its neighbours over its failure to halt the annual smog outbreaks.
PSI levels above 100 are deemed unhealthy and people are advised to reduce vigorous outdoor activity.
A cloud of greyish smoke swept across the island, accompanied by a strong smell of burning foliage.
Visibility from high-rise offices and other vantage points was virtually zero. An AFP photographer said he could hardly see the skyline from one of the city's highest points at Mount Faber.
An area in the Malaysian state of Perak had briefly tipped over to the unhealthy range for a few hours last week, according to local media.
Singapore last September closed schools and distributed protective face masks as the air pollution index soared to hazardous levels following three weeks of being cloaked in smoke from Indonesia's nearby Sumatra island.
Indonesia's Meteorology, Climatology and Geophysics Agency said on its website that the number of "hotspots" on Sumatra -- which sits across the Malacca Strait from Singapore -- had increased in the past 48 hours.
As of midnight local time on Thursday, there were 68 hotspots on Sumatra, up from 43 two days earlier, the agency said.
In the Indonesian part of Borneo island -- another area where large numbers of smog-belching fires occur every year -- there were 31 hotspots as of midnight Thursday local time, it added.
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