Ian MacKenzie, Jakarta – Ribbons of fire along irrigation channels designed to drain a huge peat bog are helping sustain noxious smog across Indonesia's Kalimantan provinces, forestry and agricultural experts said on Tuesday.

A forest research agency said the main pollution on Borneo Island was now coming from fire in a huge area of peat covering about a million hectares (2.5 million acres) that the government has been draining for a massive rice-planting project.

The drainage ditches themselves were on fire, sources said.

"A lot of what's burning is in the peat. It's incredibly hard to put out – very dirty smoke with lots of pollutants." said Neil Byron, of the Centre for International Forestry Research (CIFOR) at Bogor, near Jakarta.

One source said: "I'm told you can see ribbons of fire burning in drainage ditches they had dug."

Bush fires mainly in Sumatra and on the Indonesian side of Borneo Island have been responsible for choking haze that spread in recent months across large tracts of Southeast Asia.

Agricultural sources said there was increasing concern over starvation in the area, which straddles the borders of South and East Kalimantan provinces northwest of the town of Banjarmasin.

Sources said the fire, caused in part by clearing the peat bog, could have destroyed the rice-planting project, which has been backed by President Suharto with the aim of ensuring Indonesia's self-sufficiency in the key staple rice.

One source said the fire in the peat was creating a silicon layer impervious to water which could affect future irrigation vital for wet rice planting.

Government officials said rain had fallen sporadically on Sumatra island at the weekend, and only three airports were closed on Tuesday due to poor visibility compared to five on Monday.

"Smog has generally eased throughout Sumatra island because of the rains on Sunday. We don't have reports of rains on Kalimantan yet and smog still persists there," an official at the forest fires control bureau in Jakarta said.

On Sumatra, visibility in the town of Jambi – one of the worst hit centres in recent months – was down to 20 metres (yards).

Jakarta itself received its first heavy rainfall for months on Monday night, but the city of over 10 million people was blanketed in thick haze on Tuesday morning from its perpetual traffic pollution.

"It rained last night in Jakarta. The haze which covers the city now is the result of pollution. What we now have to pay attention to is the possibility of floods in Jakarta," the fire control bureau official said.

CIFOR's Byron said it was difficult to give an exact estimate of the area affected by fires, but he said a rough estimate was 800,000 to one million hectares (2.0 to 2.5 million acres).

The authorities have blamed plantation and forestry firms and small farmers for setting fires to clear land for planting.

Byron said the worst affected areas appeared to have been logged over forest. From an economic and ecological point of view, "most of that forest is already dead, if not cremated."

He said the El Nino upswelling of warmer water in the Pacific Ocean off South America, which affects global weather patterns, was likely to continue affecting Indonesia, now in the grip of its most serious drought in decades.

Dry winds from Australia are forcing back the massive monsoon rain clouds to the north that normally spread over the archipelago from September. Some experts say the rains may not come before the new year.