MANILA, PHILIPPINES >> Haze blown by monsoon winds from fires in Indonesia has begun affecting some areas of the Philippines and raised concerns about aviation safety and possible health risks, an official said Friday.

Landrico Dalida Jr., the deputy administrator of the Philippine Atmospheric, Geophysical and Astronomical Services Administration, said light to moderate haze was covering the southern city of Zamboanga, the central cities of Cebu and Dumaguete and the western province of Palawan. Authorities were verifying other areas that might also be affected, he said.

“It’s visible, meaning there are particles that are really coming from those areas in Indonesia and they reach us,” Dalida said.

If visibility is affected, airport officials and airlines may cancel flights because of safety concerns. Dalida advised people to wear masks if the haze worsens.

Officials of the Environmental Management Bureau, in charge of pollution prevention and control, said its regional offices have relayed information to local officials in haze-hit areas to enable them to advise the public, especially people with respiratory ailments, to stay indoors if conditions worsen.

Indonesia’s Disaster Mitigation Agency said the number of hotspots has been rising in the past week and reached 5,086 on Friday, despite government efforts to battle the fires and control the haze.

The agency recorded 1,443 hotspots in Central Kalimantan province on Borneo, an island which is divided among Indonesia, Malaysia and Brunei.

It said 99% of the hotspots were caused by deliberately set fires.

Firefighting efforts involve 52 helicopters dropping 270 million liters (71.3 million gallons) of water and 163 tons of salt for cloud seeding.

Indonesian authorities have deployed more than 29,000 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.

The six provinces which have a combined population of more than 23 million have declared an emergency since the fires began in February.

National police spokesman Muhammad Iqbal said police have arrested 349 people suspected of starting some of the fires. Those arrested could be prosecuted under an environmental protection law that provides for a maximum 10-year prison sentence for setting fires to clear land.

Forestry and Environment Minister Siti Nurbaya Bakar said the ministry is investigating 370 plantation companies suspected of intentionally setting fires for new planting, including 103 in Riau province.

She said authorities have sealed off land owned by at least 52 companies in the past week for investigation after fires were found there, including a Singaporean-based company and four firms affiliated with a Malaysian palm oil corporate group.

Indonesia’s fires are an annual problem that strains relations with neighboring countries. The smoke from the fires has blanketed parts of Indonesia, Singapore, Malaysia and southern Thailand in a noxious haze. The fires are often started by smallholders and plantation owners to clear land for planting.

Poor visibility caused by smoke has caused delays of flights at several airports in Indonesia and Malaysia and prompted authorities to shut thousands of schools in some parts of the two countries, affecting more than 1.5 million students in Malaysia alone.

Indonesia’s annual dry season fires were particularly disastrous in 2015, burning 2.6 million hectares (10,000 square miles) of land. The World Bank estimated the fires cost Indonesia $16 billion, and a Harvard and Columbia study estimated the haze hastened 100,000 deaths in the region.