Pekanbaru – The number of forest fires raging in Sumatra has increased, with winds carrying choking smoke over parts of Malaysia and slashing visibility, officials said on Friday.

The fires are a regular occurrence during the dry season in areas such as Sumatra and Borneo, but the situation has deteriorated in the last decade, with timber and plantation firms often blamed for deliberately starting fires to clear land.

The worst haze hit in 1997 and 1998, when drought caused by the El Nino weather phenomenon led to major fires. The smoke spread to Singapore, Malaysia and south Thailand and cost more than $9 billion from damage to tourism, transport and farming.

Risks for another bad period appear to have risen with the prospect of a return of El Nino this year.

An official said that 47 hot spots were recorded in Riau by Thursday, based on satellite surveillance, and temperatures were abnormally high at 35 degrees Celsius.

"There is a potential for the number of fire spots to rise and haze conditions to worsen if there is no rain," said Blucer Dolok Saribu, head of the meteorology and geophysics agency in Riau's provincial capital of Pekanbaru.

The haze is likely to remain a threat until at least August. The rainy season usually begins in September.

The city, which frequently suffers from haze, had been shrouded in unpleasant yellow-colored smoke earlier this week, although by Friday visibility had improved after some rains.

Rahmad Tauladani, a meteorological analyst in Pekanbaru, said wind had blown the haze over neighboring Malaysia, but said that no flights had been canceled so far, with visibility of 6,000 meters, above the minimum of 1,000 meters required.

In Malaysia, the haze reduced visibility in some areas near the capital Kuala Lumpur to as low as 5,000 meters, the country's Department of Environment said, with two areas recording air readings that were unhealthy.

"We are monitoring the situation. We will decide later if any action should be taken," the department's Director General Rosnani Ibrahim said.

In 2002, Southeast Asian countries signed the Asean Agreement on Transboundary Haze Pollution, but Indonesia has yet to ratify the pact.

It is illegal to carry out slash-and-burn land clearing in Indonesia, but prosecutions take time and few have stuck.

Weather expert Tauladani said current high temperatures had increased the risk that fires could spread to peatland areas.

Environmentalists are particularly concerned about an increasing trend toward converting peatland forests, which creates highly flammable areas of peat soil that produce more smoke when burned.

Indonesia has identified the Asean haze pact as one of six "crucial" bills that should be passed before the current legislative session expires at the end of September.

But with politicians distracted by the July 8 presidential election, it is unclear whether there will be time in the current term to pass it. The pact calls for signatories to work together on monitoring and combating haze.