Singapore's air quality hit the unhealthy range on Thursday afternoon, a day after the pollution index hit the highest level since 2006 on Wednesday.

According to the website of local daily the Straits Times, the three-hour Pollutant Standards Index (PSI) recording climbed to 107 as at 7p.m.local time, much higher than the 80 on Wednesday, which was the worst recording since 2006 when Singapore was last blanketed by a severe haze.

A PSI above 101 puts the air quality in the unhealthy range.

Singapore's National Environment Agency (NEA) warned that the smoke, due to fires on the Indonesian island of Sumatra, is expected to remain in the eye until at least Saturday. The NEA also advised the public to reduce vigorous outdoor activity.

Singapore's Minister for the Environment and Water Resources Yaacob Ibrahim said Singapore may register its concerns with the Indonesian authorities "more strongly" if the situations worsens.