Haze from Indonesian forest fires continued to plague Singapore on Friday (June 21) worrying workers and residents in the city state.

Singapore, which usually enjoys clear skies, saw its air quality deteriorate sharply on Monday (June 17). Its pollution standards index (PSI) soared to a record of 401 at 12pm (0400 GMT) on Friday, easily exceeding the record of 226 hit in 1997. A PSI reading above 300 is deemed "hazardous" by the National Environmental Agency.

Illegal burning of forests and other land on Indonesia's Sumatra island to clear space for palm oil plantations is a chronic problem during the June to September dry season.

On the fifth day of heavy smog, the smell of burned wood filled the air, skyscrapers were barely visible and haze hung in the tunnels that link Singapore's metro stations and shopping malls in the central core.

Singapore, a major financial centre, has sent officials to an emergency haze meeting in the Indonesian capital, Jakarta.

Prime Minister Lee Hsien Loong on Thursday (June 20) warned that the haze could persist for weeks or longer.

Air traffic controllers in Singapore gave more time for aircraft between taking off and landing at Changi Airport due to poor visibility, causing some delays.