Malaysia sent half a million face masks to the eastern state of Sarawak and closed more than 400 schools there on Tuesday as a haze spread from forest fires, officials said.

Smoke was drifting over from blazes in neighboring Indonesia, Malaysian authorities said – though Indonesia dismissed that accusation and said forest fires had also started in other countries across the region.

Singapore said its air conditions could reach unhealthy levels over the next 24 hours if the wind kept bringing in pollution, and advised anyone feeling unwell to seek medical attention.

Forest fires have raged through parts of the Indonesian island of Sumatra and the province of Kalimantan on Borneo in recent weeks, forcing the government to send in thousands of military and police to douse the flames.

Indonesia’s neighbors have regularly complained about smog caused by its forest blazes – often started by farmers trying to clear land for palm oil and pulp plantations.

Authorities closed 409 schools in Sarawak, on the Malaysian side of Borneo on Tuesday, the state education department said.

Malaysia’s national disaster management agency (NADMA) said it had sent 500,000 face masks to the state, after the air pollution index reached unhealthy levels on Tuesday.

Slash and Burn

One Sarawak district saw a “very unhealthy” pollution level of 201, while unhealthy readings were seen in five other Malaysian states on the country’s peninsular, the agency said.

Southeast Asia has suffered for years from annual bouts of smoke caused by Indonesian farmers’ slash-and-burn practices, raising worries about health and the impact on tourism.

But Indonesia’s climate agency BMKG on Tuesday disputed Malaysia and Singapore’s reports that smog in their countries was from its forest fires, chairwoman Dwikorita Karnawati told a news conference.

She said forest fires had also been spotted in Malaysia, the Philippines, Papua New Guinea, Vietnam, East Timor and Thailand. Earlier media reports that her agency had acknowledged the spread from parts of Indonesia had been based on a misinterpreted statement, she added.

Singapore recorded slightly hazy conditions on Tuesday due to persistent fires in Sumatra’s Riau and Jambi provinces, the country’s national environment agency said.

“Given the air quality forecast for the next 24 hours, healthy persons should reduce prolonged or strenuous outdoor physical exertion,” the agency said.

“Persons who are not feeling well, especially the elderly and children, and those with chronic heart or lung conditions, should seek medical attention,” it added.