The haze is expected to deteriorate today, and could even hit the high end of the very unhealthy range, or a 24-hour Pollutant Standards Index (PSI) range of 201 to 300.
This could happen if denser haze is blown in by prevailing winds from the south-east or south, said the National Environment Agency (NEA) yesterday.
Air quality is forecast to be between the high end of the unhealthy range and the mid- section of the very unhealthy range, it said.
As of 9pm last night, the 24-hour PSI was from 149 to 190.
"Haze from Sumatra and Kalimantan is still spreading to large parts of the surrounding region," said the NEA.
It advised the elderly, pregnant women and children to minimise outdoor activities, and those with chronic lung or heart disease to avoid such activities altogether.
Hazy conditions began here in end-August and looked to stay until next month, as seasonally dry weather is expected until November because of the El Nino effect. Air quality was at its worst last Friday, when the 24-hour PSI crossed into the hazardous range, or above 300.
For the next two weeks, air conditions could remain poor with lower than average rainfall. Four to six days of short thundery showers and one or two days of thundery showers with gusty winds are expected, said the NEA in a separate advisory, adding that rainfall is likely to be below normal.
Dr Erik Velasco, a research scientist from the Singapore-MIT Alliance for Research and Technology, said this is "not good news". But he added that rainfall in Singapore helps to ease the pollution only for a few hours.
"What we need is rain in Sumatra and Kalimantan."
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