Jakarta – Choking smog worsened in Indonesian parts of Borneo island on Monday but rains cleared much of the haze that had been blanketing Sumatra, officials said.

Visibility was again reduced to tens of metres (yards) in at least two of Borneo's provinces and many schools and offices remained closed as transport continued to be disrupted.

"Smoke has got thicker and thicker. I cannot even see the runway from my office window now. Visibility has reached 50 metres," Jamaluddin Hasibuan, an airport official, told Reuters from the capital of Central Kalimantan.

"We had some showers yesterday but it only made things worse because of the burning peat land the smoke rose up," he added by telephone from Palangkaraya city, some 875 km north of Jakarta.

The airport has been closed since the start of the month because of the haze and last week the governor of Central Kalimantan province ordered schools and offices to close, although several reopened on Monday.

Bouts of haze from forest fires, mainly from Indonesia's Kalimantan provinces on Borneo, have been a problem for six weeks, but residents in Sumatra's Riau province breathed easier on Monday as rain helped disperse the pollution.

"Visibility here was as low as one km ... but then there were light showers and winds...now the visibility is three km," said an airport official in the capital, Pekanbaru.

Smoke from the fires has also affected neighbouring Singapore and Malaysia although it is not as bad as in 1997 and 1998, when dense haze cost regional economies $9 billion in damage to farming, transport and tourism.

Conservationists have long criticised Jakarta for failing to protect its natural resources. Indonesia admits its laws are too weak to deal with the problem and is promising reform.