Haze from fires in Indonesia blanketing Singapore could persist for weeks or longer, Prime Minister Lee Hsien Loong said on Thursday, as the smoke drove air quality to "hazardous" levels and disrupted business and travel in the region.
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.
One Indonesian minister accused Singaporeans of acting like children, but pollution levels in the normally pristine city-state have shattered records set in 1997, raising diplomatic tensions and concerns about the economic impact.
On the fourth 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. Some residents wore surgical masks or covered their faces with handkerchiefs when they walked outside.
Singapore will suffer "an immediate hit to tourism", investment bank Barclays said, noting that retailers, hotels, restaurants, gaming and other tourism-related sectors make up about 5-6 percent of the city-state's economy.
"We think arrivals will recover quickly when the haze dissipates," it said in a report. "But the situation is fluid - prolonged hazardous conditions could affect Singapore's international reputation."
An Australian couple on holiday said they cancelled a visit to the zoo and would probably stay indoors.
"I'm never coming back to Singapore at this time of the year again," said the husband, who identified himself only as Rob.
Singapore, which usually enjoys clear skies, saw its air quality deteriorate sharply on Monday. Its pollution standards index rose to a record of 371 at 1 pm on Thursday and then swung between the "very unhealthy" and "hazardous" levels.
The pollution readings in Singapore have exceeded the peak of 226 reached in 1997 when smog from Indonesian fires disrupted shipping and air travel across Southeast Asia.
In Malaysia, the southern state of Johor was the worst affected. Air quality in the coastal town of Muar worsened in the "hazardous" category, forcing 211 schools to close.
Air traffic controllers in Singapore gave more time for aircraft between taking off and landing at Changi Airport, a major aviation hub, because of poor visibility.