Fires raging across huge areas of Indonesia are spewing more greenhouse gases into the atmosphere every day than the US economy, according to estimates from global environment watchdogs.

For nearly two months, thousands of fires caused by slash-and-burn farming have suffocated vast expanses of Southeast Asia with smog, causing respiratory illnesses to soar, schools to close, and scores of flights and some international events to be cancelled.

Much of the burning is in tropical peatlands rich in carbon but which are being drained and cleared at a rapid rate to make way for agriculture, particularly fast-expanding palm oil plantations.

The World Resources Institute, using findings from the Global Fire Emissions Database, said in a recent report that since early September carbon emissions from the fires had exceeded average US daily output on 26 out of 44 days.

The United States is the world's second-largest greenhouse gas source after China.

"The burning of tropical peatlands is so significant for greenhouse gas emissions because these areas store some of the highest quantities of carbon on Earth, accumulated over thousands of years," WRI said.

"Draining and burning these lands for agricultural expansion, such as conversion to oil palm or pulpwood plantations, leads to huge spikes in greenhouse gas emissions."

The fires and resulting region-wide blanket of smoke occur to varying degrees each year during the dry season as land is illegally cleared by burning, regularly angering Indonesia's smog-hit neighbours Malaysia and Singapore.

But experts warn the current outbreak is on track to become the worst ever, exacerbated by bone-dry conditions caused by the El Nino weather phenomenon.

The fires on the huge islands of Sumatra and Borneo are typically only brought under control by November with the onset of the rainy season.

But Herry Purnomo, a scientist at the Indonesia-based Center for International Forestry Research, told AFP this week that climatology data indicated the rainy season may be delayed this year and that the fires could last until year-end.

Indonesia earlier this month agreed to accept international help after failing for weeks to douse the fires manually and last week launched its biggest fire-fighting push yet.

Thirty-two planes and helicopters -- including six aircraft from Singapore, Malaysia and Australia -- were deployed to back up more than 22,000 personnel on the ground.

Malaysia, which in recent weeks has repeatedly ordered school closures across several states as a health precaution, did so again on Wednesday for the third straight day as pollution levels climbed.

Air quality was in the "very unhealthy" range near the capital Kuala Lumpur under the government's rating system, with much of the rest of the country experiencing "unhealthy" air.