Jakarta – Smoke haze attributed to underground fires continued to choke the city of Pontianak on Indonesian Borneo Thursday, local officials said.

"It is getting worse in the evenings, with visibility down to between 50 and 400 meters but by 8am, it gets better at around 1,000 metres" said Muhammad Chaeran of the meteorology office in Pontianak. The low visibility, he said, continued to delay incoming flights at the Supadio airport of Pontianak, with the earliest landing at around 9am.

Chaeran blamed the thick smoke that cast the pall of haze over Pontianak on smouldering subterranean fires in layers of peat. "The thing is that satellite imaging does not show a substantial number of hot spots in the area," Chaeran said.

The reason, he suspected, is that the smoke comes from undergound fires in the peat layers, which release only smoke on the surface without giving noticeable "hot spot" readings. Hot spots are areas of high temperature detected by satellite that usually indicate surface fires.

Nurul Irsadi of the West Kalimantan Environmental Impact Management Agency, said that only 19 hot spots were detected by satellite in West Kalimantan on Wednesday, two of them near Pontianak and six in the neighbouring Ketapang district.

Irsadi said that the air quality in Pontianak was dangerous, registering a high ash and debris content between 9pm and 2am every night. "It ranges between unhealthy and healthy until 8am and usually healthy after that," Irsadi said. In Medan, the capital of the province of North Sumatra, the haze that has blanketted the city for the past days had noticeably thinned by Thursday thanks to stronger winds, a local meteorolgy office official there said. "Visibility is now at 8,000 metres, much better than in the previous days," she said.

Jakarta blames the haze on land clearing by fire by small farmers and large timber and plantation companies. The government has banned plantations from using fire to clear dense jungle, but has been helpless in enforcing the ban on small farmers across the country.

In 1997-1998 forest and ground fires in Sumatra and Borneo sent thick smoke over the skies of western Indonesia for months that also spilled to neighbouring countries, causing serious health and traffic hazzards and disrupting airline schedules in the region. The choking haze then caused an estimated 9.3 billion dollars in economic losses.