Raging blazes in Indonesia have been spewing toxic haze over neighbouring Singapore and Malaysia, with Jakarta deploying tens of thousands of personnel and water-bombing aircraft to tackle them.

The Indonesian fires are an annual problem during the dry season when farmers use illegal slash-and-burn techniques to clear land for agriculture, but this year's are the worst since 2015.

Thick smog had been hanging over Singapore since last weekend, pushing air quality to unhealthy levels, obscuring the waterfront skyline and sparking fears that Sunday's showpiece F1 night race may be knocked off track.

But on Friday the skies were largely clear, with only a light haze over the city, while air quality improved to a "moderate" level of around 60 on the National Environment Agency's scale.

A reading between 101 and 200 indicates unhealthy air quality.

The wind direction could however still change ahead of the weekend and push smog back over Singapore.

F1 organisers say they have a contingency plan if the haze worsens and have been stocking up on face masks to protect against pollution which spectators can buy at the circuit.

Indonesia and Malaysia have been worst affected by haze from the fires, which are burning on Indonesia's Sumatra island and the Indonesian part of Borneo. Borneo is shared between Indonesia, Malaysia and Brunei.

Thousands of schools have been forced to close in both countries amid mounting health concerns, while several airports have shut in Indonesian Borneo due to poor visibility. 

People have been dashing to buy face masks in affected areas and there has been an increase in reports of respiratory illnesses and conditions such as dry and itchy eyes.

On Friday more than 2,600 schools were closed in Malaysia, education authorities said.

The worst-affected state was Sarawak, on Borneo, where more than 1,000 schools were shuttered and air quality reached "hazardous" levels in one area bordering Indonesia, according to an official index.

Unleashing rain 

Indonesia meanwhile had some success in "cloud-seeding" -- inducing rain using chemicals sprayed from planes -- and produced a downpour over hard-hit Riau province on Sumatra, disaster agency spokesman Agus Wibowo said.

Over 2,800 "hotspots" -- areas of intense heat detected by satellite indicating a likely fire -- had been sighted in two provinces of Indonesian Borneo alone although the number had dropped in Riau, he said. 

Indonesia insists it is doing all it can and has deployed about 29,000 personnel from a range of government agencies nationwide in an effort to douse the blazes, many of which burn underground in carbon-rich peat.

Almost 250 people have been arrested on suspicion of activities that led to the blazes. 

Experts say the fires are unlikely to be extinguished until the onset of the rainy season in October and have raised fears of a repeat of the 2015 crisis, the worst smog outbreak for about two decades.

In many places the current haze "is comparable to 2015 at the same time of the year," Robert Field, a scientist from NASA Goddard Institute for Space Studies, who tracks Indonesia's fire seasons, told AFP.

A major problem for firefighters "is that once underground and into the peat (the fires) are very difficult to put out and have, for all intents and purposes, an inexhaustible supply of fuel," he said.

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