Thick white smoke from Indonesian slash-and-burn farming enveloped Malaysia's capital and other areas Sunday, triggering school closures for the following day as weeks of choking haze showed no sign of abating.

Pollution readings in Kuala Lumpur soared into the "very unhealthy" territory in the Malaysian government's hourly air-quality index.

The Ministry of Education ordered schools shut on Monday in the capital and three states due to health concerns, the second time this month it has had to issue such an order.

Malaysia, Singapore and large expanses of Indonesia have suffered for weeks from acrid smoke billowing from fires on plantations and peatlands that are being illegally cleared by burning.

The fires are located on Indonesia's huge islands of Sumatra and Borneo.

The crisis grips the region nearly every year during the dry season, but the current outbreak is one of the worst and longest-lasting in years.

Authorities have said tens of thousands of people in the three countries have been forced to seek medical treatment for respiratory problems, and that dozens of flights have been cancelled or delayed due to poor visibility.

Indonesia has faced pressure from its neighbours to address the problem since it first emerged nearly 20 years ago.

- Smouldering dispute -

But the issue has persisted, especially as plantations expand to meet rising global demand for products like palm oil, a key ingredient in a vast range of everyday consumer products.

Singapore on Friday ordered rare school closures across the city-state as air reached "hazardous" levels there, with Environment Minister Vivian Balakrishnan saying the problem has lasted "for far too long".

"This is not a natural disaster. Haze is a man-made problem that should not be tolerated. It has caused major impact on the health, society and economy of our region," he said in a statement.

The Singapore government also said Friday it had launched legal action against five Indonesian companies blamed for the fires, including multinational Asia Pulp and Paper (APP), which could lead to massive fines.

Air quality in Singapore improved Sunday, dropping below the "unhealthy" mark.

An airport just outside Kuala Lumpur closed temporarily on Saturday afternoon as visibility dropped to less then 400 metres (yards).

That forced at least 20 flights to be cancelled, according to Malaysian media reports.

Other reports on Sunday said air service between the Malaysian city of Kuching and Indonesia's Pontianak -- both of which are on Borneo -- was halted until further notice.

Indonesian National Disaster Management Agency spokesman Sutopo Purwo Nugroho said Saturday that "fires continue to rage" despite a push to extinguish the blazes by more than 25,000 military, police and other personnel.

He said new fires were cropping up, while some that were previously extinguished had flared anew or had been deliberately re-ignited.

Nugroho also said pollution readings in several Indonesian cities were at hazardous levels, and that nearly 168,000 people in the country had sought medical treatment for respiratory ailments.

Indonesia had earlier declared a state of emergency in Sumatra's hard-hit Riau province.

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