Haze from new deliberately set fires blankets parts of Indonesia’s Sumatra island after days of improving air quality.

Thick, noxious haze from new deliberately set fires blanketed parts of Indonesia’s Sumatra island on Monday after days of improving air quality, causing school closings and flight delays.

National Disaster Mitigation Agency spokesman Agus Wibowo said nearly 1,200 fires were burning, more than double the number in past weeks as authorities managed to seed clouds to induce rain in several affected areas on Sumatra and Borneo islands.

Wibowo said South Sumatra province had the largest number of detected fires, with nearly 700 hotspots, prompting authorities to shut most schools in Palembang, the province’s capital, to protect children.

Ari Subandri, the general manager of Airnav Indonesia in Palembang, said poor visibility caused delays at the city’s main airport.

Haze from Indonesian fires, often set to clear land for planting, is an annual problem for Southeast Asia.

The fires are often started by smallholders and plantation owners to clear land for planting.

Many areas of Indonesia are prone to rapid burning because of the draining of swampy peatland forests for pulp wood and palm oil plantations.

Wibowo said seven helicopters dropped 66 million litres (17.4 million gallons) of water on Monday over South Sumatra province.

He said firefighting measures included 14 tons of salt to induce rain in anticipation of worsening fires.

This year alone, at least 52 helicopters have dropped more than 371 million litres (98 million gallons) of water and 255 tons of salt for cloud seeding as part of the firefighting efforts in six provinces that have declared emergencies. The provinces have a combined population of more than 23 million.

Record Indonesian forest fires in 2015 spread haze across a swath of Southeast Asia, and according to a study by Harvard and Columbia universities, hastened 100,000 deaths.