Thousands of schools were shuttered across Malaysia and Indonesia on Thursday, affecting at least 1.7 million pupils, officials said, as toxic haze from rampant forest fires sent air quality plummeting.

Nearly 2,500 schools were ordered to shut their doors in Malaysia — including nearly 300 in the smog-hit capital Kuala Lumpur — over soaring health concerns sparked by toxic haze from out-of-control blazes in Indonesia's Sumatra and Borneo islands.

Indonesia said hundreds of schools in hard-hit Riau province on Sumatra would also be shut on Thursday, with 800 closed in one district alone, while about 1,300 were shut in its Central Kalimantan province on Borneo.

The closures affected at least 1.7 million students in Malaysia. It was not clear how many pupils were forced to stay home in neighbouring Indonesia.

Jakarta is deploying thousands of security forces and water-bombing aircraft to tackle the blazes, mostly started by illegal fires set to clear land for plantations.

The fires belch smog across Southeast Asia annually, but this year's are the worst since 2015 and have added to concerns about wildfire outbreaks worldwide exacerbating global warming.

Thursday's school shutdown marked the first mass closure in Kuala Lumpur as air quality deteriorated to "unhealthy" or "very unhealthy" levels on an official index in many parts of peninsular Malaysia, to the east of Sumatra, with the capital's skyline shrouded by dense smog.

Malaysia's Sarawak state, on Borneo, was also smothered in toxic haze, as the fires hike diplomatic tensions.Borneo is divided between Indonesia, Malaysia and Brunei.

A growing number of Malaysians were suffering health problems due to the haze, with authorities saying there had been a sharp increase in outpatients at government hospitals — many suffering dry and itchy eyes.

Air quality was in the "unhealthy" range across Singapore on Thursday morning, according to the National Environment Agency, as the city state's environment minister called it a "major setback" in the fight against climate change.

"The forest fires in Indonesia and the resulting haze have affected the health and well-being of people in Indonesia and the ASEAN (Association of Southeast Asian Nations) region," Singapore's environment chief Masagos Zulkifli said on social media.

"It is regretful that so many lives and livelihoods have been impacted.

"The amount of carbon emissions generated from the fires will present a major setback to the global fight against climate change," he added.

The haze crisis comes as Singapore gears up to host a Formula One motor race on Sunday.

Poor visibility closed several airports in the Indonesian part of Borneo, and scores of flights have already been diverted and cancelled in the region in recent days due to the smog.

Authorities have tried to induce rain through a technique called cloud seeding in a bid to extinguish the fires in Indonesia.

But the onset of the rainy season, which usually starts in October, could be the only thing able to douse the blazes.