Firefighters and volunteers ride in a truck as they look for fires to put out amid thick haze on the outskirts of Palangkaraya, in Indonesia's central Kalimantan province.

AFP/Jakarta

The death toll from acrid haze blanketing parts of Indonesia has climbed to 19, a minister said on Wednesday, almost double the previous figure as the crisis from widespread forest fires worsens.

For nearly two months, thousands of fires caused by slash-and-burn farming in Indonesia have choked vast expanses of Southeast Asia, forcing schools to close and scores of flights and some international events to be cancelled.

An estimated half a million people have suffered respiratory illnesses since the fires started in July. Indonesia's disaster agency previously stated the fires had killed 10 people, some of whom died while fighting the blazes and others from the pollution.

But the country's social minister confirmed on Wednesday the death toll had risen.

"As of this morning, there are nineteen people who have died from the effects of haze," Khofifah Indar Parawansa told reporters in Jakarta.

The victims were all from Sumatra and Kalimantan - Indonesia's half of Borneo island - where fires have been deliberately lit by farmers wanting to quickly and cheaply clear land.

President Joko Widodo plans to tour the worst-hit regions later this week, having cut short his visit to the US to deal with the crisis.

Three warships are on standby in Kalimantan in case a large-scale evacuation is needed, with temporary shelters being rapidly built to house those fleeing the toxic smog.

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.

Indonesia's security minister Luhut Panjaitan also admitted on Wednesday the country's weather agency had failed to predict this year's El Nino would be more severe than 1997, when out-of-control fires sent pollution soaring to record highs in an unprecedented environmental disaster.