Air quality in Singapore yesterday reached its worst level since the start of the year, with the haze affecting visibility in some areas and leaving a smell of smoke.

The Pollutant Standards Index's (PSI) three-hour reading had gone from "good" - 31 at 9am - to "moderate" by 6pm, hitting a high of 71 at 8pm and falling to 61 two hours later.

On Thursday, the National Environment Agency (NEA) said that the transition from the Northeast Monsoon to the inter-monsoon period in the second half of March "may pose some risk of transboundary haze affecting Singapore".

The Riau province of Sumatra has been shrouded in dense haze over recent days as farmers set fires to clear land during the dry spell. On March 5, NEA detected six hotspots in Sumatra, Indonesia and 47 hotspots in peninsular Malaysia. But it added that the low hotspot count for Sumatra was due to cloud cover and partial satellite coverage.

Singapore is in the midst of one of its longest spells of dry weather in years, with barely any rain over the past month and a half.

NEA said it will continue to monitor the situation and provide updates. These can be accessed from NEA's Facebook and Twitter accounts and also at the haze microsite at www.haze.gov.sg.