Visibility in parts of Borneo was reduced to 50m while Singapore recorded its worst pollution levels since 1997.

Flights were cancelled, cars put their headlights on in the middle of the day, and Singapore warned citizens against taking exercise outdoors.

Indonesians use the fires for land clearance despite a government ban.

Visibility was down to 50m in Central Kalimantan, on Indonesia's part of Borneo island.

Environmental agencies reported from 500 to 2,000 fires burning in the flammable peat soils of the region.

"The worst situation is in Central Kalimantan now. Most areas in the province contain peat," Malaysian forest fire chief Purwasto told Reuters news agency.

"We cannot estimate the extent of the fires now."

Singapore's environment agency issued a health advisory yesterday as a thick haze from illegal fires in Indonesia choked the city-state and surrounding region.

Singapore's pollution index at 0800 GMT was 128, compared with 80 on Friday. A reading above 101 is considered unhealthy.

"Persons with existing heart or respiratory ailments should reduce physical exertion and outdoor activity. The general population should reduce vigorous outdoor activity," the National Environment Agency advised.

In neighbouring Malaysia, pollution also worsened as readings from the department of environment showed air quality at unhealthy levels in 20 areas of the country, up from 12 on Friday.

"The smoke haze situation has deteriorated overnight. The prevailing winds have remained southerly to southwesterly overnight and they have brought smoke haze from southern Sumatra to Singapore," a statement from the city-state's environment agency said.

At 0600 GMT yesterday, Indonesia's ministry of forestry reported 2,000 hotspots in Kalimantan, mostly in Central Kalimantan province on Borneo island. On Sumatra island only 291 hotspots were burning, the ministry said.

Poor visibility of 50 to 100 metres (yards) in Palangkaraya, capital of Central Kalimantan province, stopped all flights in the morning, said Hidayat, an official from the local meteorological office.

A spokesman for Singapore's Changi Airport on Saturday said the haze had not affected operations at the regional aviation hub.

On Friday flights were disrupted in Malaysia's Sarawak province on Borneo island.

In Indonesian Borneo, hundreds of firefighters, aided by police and volunteers, have been trying to douse the illegal forest fires.

Residents in the southern Thailand provinces of Narathiwat and Yala, which border Malaysia, reported clear skies and fine visibility on Saturday.

Malaysia's meteorological department said it expected conditions to improve over the weekend in Sarawak as well as Sabah state and the country's north.

But light showers expected in the Kuala Lumpur area and the country's south would offer little respite there.

"It won't help very much," a department spokesman told AFP.

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

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

"The fires are seasonal and very predictable, but the government never implements effective measures to prevent and manage them," said Nordin, of the Palangkaraya-based environmental group Save Our Borneo.