JAMBI, Indonesia - Huge fires are raging across vast swathes of Indonesia's rainforests -- some of the world's biggest -- with toxic smog shutting hundreds of schools in Southeast Asia, officials said Tuesday.

Massive jungle areas in Sumatra and Borneo islands are ablaze as thousands of personnel battle to quell the fires, frequently started to clear land for crop plantations.

Burning forests to make way for farming is also thought to be behind the enormous fires currently ripping through the Amazon in South America, and experts believe they could have a serious impact on the global climate.

In Indonesia the number of hotspots -- areas at risk of fires -- has soared in parts, including on Borneo which the country shares with Malaysia and Brunei.

Air quality has dropped to "unhealthy" levels in and around Kuala Lumpur, according to the government's air pollutant index, and the skyline has been shrouded in thick smog.

The smell of burning foliage filled the air, and residents were suffering respiratory problems and complained of itchy and sore eyes.

"It makes your eyes hurt and causes breathing problems," Indonesian tourist Indah Sulistia told AFP in Kuala Lumpur.

"The haze also creates problems for snapping photos," she added.

Haze also hung over Singapore, while residents in parts of southern Thailand were advised to wear face masks this week.

'REALLY SCARY'

Around 400 schools were closed Tuesday in nine districts of Malaysia's Sarawak state on Borneo, with more than 150,000 students affected, according to the local education department.

In neighboring Indonesia's Jambi province, on Sumatra, some kindergartens will be closed until Friday, while elementary and junior high schools are also temporarily shut, according to local authorities, who did not give exact numbers.

Jambi mayor Syarif Fasha urged residents to wear face masks while Malaysia's national disaster management agency said it has secured half a million masks, which will be sent to the Sarawak state disaster committee.

"This is really scary," said Jambi resident Atiah, who goes by one name like many Indonesians.

"I'm coughing and keep having trouble breathing -- I don't know what this is going to mean for my respiratory health."

On Monday, Malaysia said it was preparing to carry out cloud seeding to induce rain and clear the air by releasing certain chemicals into the clouds -- although some experts have questioned its efficacy.

Indonesian environmentalists called for a crackdown on land-clearance burning.

"We hope the government will enforce the law against negligent people who have let their land burn," said Rudi Syaf, director of green group Indonesian Conservation Community.

Many of the worst fires occur in carbon-rich peat, which is highly combustible once drained to make way for agricultural plantations.

Jakarta has deployed thousands of extra personnel since last month to prevent a repeat of 2015 fires, which were the worst for two decades, choking the region in haze for weeks and setting off a diplomatic spat.

A US study said the 2015 fires may have caused more than 100,000 premature deaths due to respiratory and other illnesses.

Under pressure from neighbors, Indonesian leader Joko Widodo last month warned that officials would be sacked if they failed to stamp out forest fires.

The number of hotspots with medium-to-high potential to break out in blazes soared nearly sevenfold to 6,312 over a four-day period this month, according to Indonesia's national disaster agency.