Hazy conditions are expected to persist for the next few days, said the National Environment Agency (NEA) on Friday, adding that it will monitor the situation closely.

This is due to drier weather conditions in the region in recent days, leading to an increase in hotspot activities observed mainly over central Sumatra. A total of 85 hotspots were detected on June 6, but this decreased to 22 on Thursday due to "more cloud cover over Sumatra".

"The haziness and burning smell are from the fires in Sumatra, brought over by prevailing winds blowing from the southwest or west during the current Southwest Monsoon season," said the NEA in a statement.

The three-hour Pollutant Standards Index (PSI) stood at 85 at 6pm on Friday, which is slightly lower than the 88 recorded at 5pm. This is at the higher end of the "moderate range" of 51 to 100. Meanwhile, the latest 24-hour PSI average, measured at 4pm on Friday, stood between 59 and 65, also within the "moderate" range.

The last time air quality here deteriorated to unhealthy PSI levels - when the index passes 100 - was in 2010, also caused by haze attributed to forest fires in Indonesia.

The public may access PSI updates through http://weather.nea.gov.sg or NEA's Twitter account @NEAsg. They may also call 1800-225-5632 or use NEA's myEnv mobile application on their smartphones.