JAKARTA, Indonesia - Indonesian authorities closed an airport on Sumatra island on Friday due to poor visibility caused by smoke from raging fires burning through peatland, while schools in several provinces and in neighbouring Malaysia closed due to the hazardous haze.

Fluctuating visibility that at times was just 300 metres (yards) forced airlines to postpone flights to the main airport in Pekanbaru, the capital of Riau province, airport official Yogi Prasetyo said. Poor visibility also caused delays at an airport in Dumai, another city in Riau.

Worsening haze caused more schools in Riau and Jambi provinces to close, education officials said. Health offices in both provinces said more than 300,000 people have suffered respiratory illnesses since the haze began, and officials said more people have been seeking medical help in recent weeks.

Nearly every year, Indonesian forest fires spread health-damaging haze across the country and into neighbouring Malaysia and Singapore. The fires are often started by smallholders and plantation owners to clear land for planting.

Some schools in Malaysia’s central Selangor state shut on Friday, affecting 45,000 students, after air pollution levels spiked, the state Education Department said. Earlier this week, hundreds of schools were shut for a day in eastern Sarawak state, which neighbours Indonesia’s West Kalimantan province where more than 25,000 hectares (60,000 acres) of land were deliberately burned.

Officials have said Malaysian Prime Minister Mahathir Mohamad would write to Indonesian President Joko Widodo to press for urgent action to tackle the cross-border haze.

Indonesian Forestry and Environment Minister Siti Nurbaya Bakar sparked anger this week by disputing that the smoke was coming from Indonesia, noting that hotspots were also detected in Malaysia’s Sarawak state.

“I think there was incorrect information, because the Indonesian government is trying systematically to solve this as well as possible,” Bakar told reporters in Jakarta.

Malaysian Environment Minister Yeo Bee Yin responded immediately, telling Indonesia “not to be in denial.” She cited data from the ASEAN Specialized Meteorological Center that showed the haze impacting parts of Malaysia originated in Indonesia.

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

The Indonesian Disaster Mitigation Agency said Friday there were more than 5,000 hotspots nationwide, including on Sumatra and Borneo islands, leading to very poor air quality in six provinces with a combined population of more than 23 million. The provinces have declared a state of emergency.

It said more than 40 helicopters have dropped about 240 million litres (63 million gallons) of water and more than 160,000 kilograms (350,000 pounds) of salt for cloud seeding as part of the firefighting efforts.

Indonesian authorities have deployed more than 9,000 people to fight the fires, which have razed more than 328,700 hectares (812,000 acres) of land across the nation, with more than half in the provinces of Riau, Jambi, South Sumatra, West Kalimantan, Central Kalimantan and South Kalimantan.

Malaysia’s government has banned open burning and conducted cloud-seeding in Sarawak on Thursday to induce rain to clear the choking haze.

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.

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