Choking smoke from forest fires hung over parts of Indonesia's Sumatra island on Friday, forcing a delay in flights, and prompting fears that conditions could worsen because of lack of rain, officials said.

About 450 hot spots have been detected across Indonesia, and forestry officials have warned that the number could exceed last year's total of 35,000 as the dry season this year is likely to be marked by less rain than usual.

"This morning two planes were delayed for half an hour because the visibility was only 200 metres because smoke and smog shrouded the airport," said Slamet Riyadi, analyst at Riau Meteorology and Geophysics Agency, referring to the airport in Pekanbaru, the provincial capital of Riau.

The number of hotspots had risen to 393 in West Kalimantan island on Borneo island, although the number in Sumatra had decreased after light rain late on Thursday, officials said.

Indonesia's neighbours have grown increasingly frustrated by the annual fires, most of which are deliberately lit by farmers, or by timber and plantation firms, to clear land for cultivation.

Depending on wind patterns, the smoke regularly blows across nearby Singapore, Malaysia and Thailand, causing a health hazard and threatening tourism.

Most of the fires so far this year were on land being used for cultivation, although some were in forested areas, said Sonny Partono, head of forest fire control at the forestry ministry.

The ministry has said it is deploying 1,500 personnel to fight fires this year, assisted by community groups.