JAKARTA, Indonesia (AP) — Authorities shut most schools in parts of Indonesia's Sumatra island to protect children from a thick, noxious haze as deliberately set fires burned through peatland forests, officials said Wednesday.

Indonesia's Disaster Mitigation Agency said more than 3,600 fires have been detected on Sumatra and Borneo islands by weather satellites, leading to very poor air quality in six provinces with a combined population of more than 23 million.

Nearly every year, Indonesian forest fires spread health-damaging haze across the country and into neighboring Malaysia and Singapore.

Authorities have deployed more than 9,000 people to fight the fires, which have razed more than 162,000 hectares (400,000 acres) of land in the provinces of Riau, Jambi, South Sumatra, West Kalimantan, Central Kalimantan and South Kalimantan.

Riau provincial secretary Ahmad Syah Harofie said air pollution hit hazardous levels in the provincial capital, Pekanbaru, and was very unhealthy in many other areas. Several thousand schools in the capital and three other cities and districts have been closed since Tuesday.

He said nearly 300,000 people in the province have suffered respiratory illnesses since January, when it and the five other provinces declared states of emergency due to the forest fires.

The fires burned parts of the Tesso Nilo National Park in Riau, home to about 140 endangered wild elephants, according to Edward Sanger, the local disaster agency spokesman.

Thousands of Muslims, many wearing face masks to protest themselves from the smoke, joined mass prayers for rain in Pekanbaru.

Authorities in Jambi, another province on Sumatra island, also ordered several thousand schools to close.

About 8,000 people have suffered respiratory problems in the past week alone, according to Jambi's health office.

Poor visibility caused delays at several airports on Wednesday.

The Disaster Mitigation Agency said satellites have detected about 5,062 hotspots nationwide on Wednesday morning, with the largest number in Central Kalimantan on Borneo island. It said 37 helicopters have dropped nearly 240 million liters (63 million gallons) of water as part of the firefighting efforts.

The number of hotspots declined significantly to 2,388 on Wednesday afternoon, the agency said.

The haze is an annual problem for Southeast Asia. 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.

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.

In neighboring Malaysia, the largest city, Kuala Lumpur, and the government administrative center of Putrajaya were among the areas shrouded in thick smog Wednesday. On Tuesday, hundreds of schools in eastern Sarawak state bordering Indonesia's Kalimantan province were closed for a day after air quality spiked to unhealthy levels.

Malaysian authorities plan to conduct cloud-seeding activities to induce rain to ease the haze. The government has said it will press Jakarta to take immediate action to put out the burning forests and ensure the fires won't occur again.

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Associated Press writer Eileen Ng in Hong Kong contributed to this report.