Singapore's three-hour average Pollutant Standard Index eased to 37 at 1000 GMT from Saturday's peak of 150, a nine-year high. A reading of 0-50 is good while anything between 101-200 is unhealthy.

But the National Environment Agency said in a statement a change in wind direction after late afternoon Sunday could see the readings rise again.

"The winds will then change to southwesterly direction and bring in the smoke haze from Sumatra to Singapore," the agency said.

Singapore has been hit by the fog-like haze, which has a burning smell, for several days and issued a health advisory Saturday suggesting people should cut outdoor activity.

Singapore's Environment Minister Yaacob Ibrahim said the city-state had told Indonesia that it was necessary to control the forest fires, which are lit to clear land, quickly.

"We told our Indonesian colleagues of our concerns that the air quality has already deteriorated ... and we impressed upon the Indonesian officials to suppress the forest fires as quickly as possible," he was quoted as saying in the Sunday Times.

He said Singapore had offered to help Indonesia douse the fires through cloud-seeding to induce rain and to assist Indonesian farmers in land clearing, but Jakarta had yet to take up the offer.

Meanwhile haze was thickening Sunday in the Indonesian part of Borneo island as the number of forest fires there was reportedly increasing, especially in the province of central Kalimantan, officials said.

"It is definitively much denser than yesterday (Saturday) especially since we have a high level of humidity here," Hidayat, the head of the meteorology station in Palangkaraya, central Kalimantan's capital, told AFP.

He added that visibility since daylight had remained around just 20 meters.

Data from satellite images taken Saturday afternoon showed a total of 1,710 hotspots -- areas with high temperatures indicating fires -- in central Kalimantan, up from the previous day's 1,160. The rest of Kalimantan had 260 hotspots.

Malaysia, where the haze caused air quality to become unhealthy in some regions, called on Indonesia to ratify a regional anti-haze agreement, a report said Sunday.

Malaysia's natural resources and environment minister Azmi Khalid said it was unclear why Indonesia had dragged its feet on ratifying the deal.

"Only when Indonesia ratifies the agreement can member countries set up a centre so that prevention and extinguishing of fires can be done easily," Nazmi was quoted as saying in the Star daily.

The Association of Southeast Nations in 2002 signed a trans-boundary agreement to tackle haze pollution, which blights the region annually, but critics say action has been slow to follow.

The eastern state of Sarawak on Borneo island, the worst-affected by the haze, has reported a spike in respiratory ailments including asthma. Airline traffic there was also disrupted last week.

Indonesia's annual burn-off causes a haze that typically smothers parts of Malaysia, Singapore and Thailand as well as Indonesia itself.

In 1997 the fires caused a thick regional fog, which sent Singapore's pollution index to an all-time high of 226.

The Indonesian government has outlawed land-clearing by fire but weak enforcement means the ban is largely ignored.