Jakarta (AFP) - The forest fires blanketing Southeast Asia in choking haze are on track to become among the worst on record, NASA has warned, with a prolonged dry season hampering efforts to curb a crisis that has persisted for nearly two decades.

Malaysia, Singapore and large expanses of Indonesia have suffered for weeks from acrid smoke billowing from fires on plantations and peatlands that are being illegally cleared by burning.

Though the crisis grips the region nearly ever year during the dry season, scientists predict the current outbreak could surpass 1997 levels, when out-of-control forest fires sent pollution soaring to record highs in an environmental disaster that cost an estimated USD$9 billion.

If the forecasted dry conditions extend, the region could be enveloped in even denser smog, exacerbating a crisis that has seen flights grounded, schools closed and tens of thousands of people seek medical treatment for respiratory problems.

“Conditions in Singapore and southeastern Sumatra are tracking close to 1997,” Robert Field, a Columbia University scientist based at NASA’s Goddard Institute for Space Studies, was quoted as saying by the US science agency.

“If the forecasts for a longer dry season hold, this suggests 2015 will rank among the most severe events on record.”

Pressure on Indonesia from its neighbours has intensified this year as the fires have raged on, with Jakarta deploying more than 20,000 troops, police and other personnel to fight the fires through waterbombing and chemically-induced rainfall.

An El Nino weather system has made conditions drier than usual in Indonesia, but authorities are hoping much-needed rains will arrive within a month to finally douse the blazes.

It could be too little too late in Malaysia, where farmers have complained of poor yields due to the haze, and in Singapore where the government has launched legal action against companies blamed for farm and plantation fires.

But in more bad news, the worst of the smog shrouding the region could be yet to come.

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Herry Purnomo, a haze expert at the Indonesia-based Centre for International Forestry Research, told AFP the dry season was not expected to peak in Sumatra until next month, when more smoke was predicted.

"It's not over yet," he said, adding the economic cost of the "horrendous" fires would be as bad as 1997.

Authorities worry air quality in Singapore, which improved Friday to the lower end of the “unhealthy” range after heavy rains overnight, could worsen again depending on the wind direction from Sumatra.

In Malaysia, where weeks of fog-like white-gray smoke has forced repeated large-scale school closures out of health concerns, there's fears the prolonged dry season could spark fires in Malaysia, compounding the disaster.

"That will be a double catastrophe," said Lim Teck Wyn, who organised a protest march to Indonesia’s embassy in Kuala Lumpur over the haze.

The annual haze crisis has persisted despite all efforts, especially as plantations expand to meet rising global demand for products like palm oil, a key ingredient in a vast range of everyday consumer products.

The fires smoulder beneath the surface of carbon-rich peatlands, feeding off vast quantities of fuel, making them extremely difficult to curb as millions of tonnes of greenhouse gas emissions are released into the atmosphere.

The NASA-linked Global Fire Emissions Database has estimated around 600 million tonnes of greenhouse gases have been released as a result of this year's fires - roughly equivalent to Germany's entire annual output.