Indonesia has been fighting wildfires for several weeks across parts of Borneo and Sumatra, with thick toxic haze spreading across the country and into neighbouring Malaysia and Singapore.

Around 350 members of Japan's Self-Defense Forces arrived last week in Borneo's hard-hit West Kalimantan province, with three helicopters on hand to assist with firefighting efforts, the mission's public affairs officer Takeshi Koga said .

Their operation was planned to start on Sunday but has been scuppered by low visibility, Koga said, adding that their water-bombing operation eventually started on Wednesday.

Thursday's flights "were all cancelled due to poor visibility" as smog reduced the range of sight to less than three kilometres (1.9 miles), he said.

Indonesia regularly battles wildfires during the dry season, which typically runs from July to October.

But the blazes this year have been particularly bad in part because of the El Nino climate phenomenon expected to be the strongest since comparable records began four decades ago.

Experts also blame climate change, decades of forest degradation and slash-and-burn land-clearing practices that have changed the naturally fire-resistant landscape.

The Indonesian government has said more than 202,000 hectares of land burned countrywide between January and July.

The Nusantara Atlas project put the figure at close to 300,000 hectares over the same period, and last week estimated that the total area affected, including August, was nearly 896,000 hectares.