PEKANBARU, Indonesia (Reuters) – Indonesia closed more schools on Monday in parts of the islands of Borneo and Sumatra, as well as providing oxygen at health centres to help treat some of the thousands suffering respiratory infections due to choking smoke from forest fires.

Indonesia and neighbouring countries in Southeast Asia are regularly hit by smoky haze from slash-and-burn clearances of forests for timber and palm oil plantations, but conditions this year have been the worst since 2015 due to an El Nino weather pattern causing an extended dry spell.

In recent days, thick smog from fires has been drifting across cities in Borneo and Sumatra. Indonesia’s disaster mitigation agency says more than 328,000 hectares of forests and peatlands have been burnt since January.

The air pollution index in Palangka Raya, the capital of Central Kalimantan province on Borneo, has been at “dangerous” levels for days, data from Indonesia’s Environment and Forestry Ministry showed. The air quality in other areas, such as Jambi and Riau provinces in Sumatra, also hit “unhealthy” levels.

Schools in two cities in Central Kalimantan have been closed this week due to concerns over the pollution, while schools in Riau have also been shut.

West Kalimantan Governor Sutarmidji had extended school closures in his province until Wednesday, according to a circular issued on Monday.

Mimi Nazir, the head of Riau’s health agency, said the Sumatran province had opened 12 temporary centres in the capital Pekanbaru to treat people suffering respiratory infections.

“If people need fresh air, they can come to the houses. There’s oxygen, medical staff, and basic and emergency medicines,” Nazir said by telephone, adding that if people needed further treatment they could be referred to hospital.

Acute respiratory infections have affected 11,758 people in Palangka Raya, 15,346 in Riau and 15,047 in Jambi, according to data from Indonesia’s Health Ministry.

The ministry distributed more than 1.2 million masks in impacted areas as well as 1,000 vials of respiratory medicine.

Poor visibility due to the smoke affected 11 airports across Sumatra and Indonesian parts of Borneo on Monday, causing 10 flight to be cancelled, 50 delayed and two diverted, air traffic control provider AirNav said in a statement.

Indonesia’s flag carrier Garuda <GIAA.JK> said in a statement on Monday it had cancelled at least 15 domestic flights to cities in Indonesian parts of Borneo until Thursday due to concerns over limited visibility.

(Reporting by Reuters stringer in RIAU, Stanley Widianto and Tabita Diela in JAKARTA; Editing by Ed Davies and Alex Richardson)