Indonesia plans to use weather changing technology to try to unleash torrents of rain and extinguish raging fires on Sumatra island that have cloaked neighbouring Singapore in thick haze, an official said Wednesday.

The city-state, which is home to 5.3 million inhabitants, has been pressing Jakarta to take action to put out the blazes, which have pushed air pollutant levels on the island to a 16-year high.

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Indonesian forestry ministry official Raffles Panjaitan said the government planned to use a technology called “cloud-seeding” to try and put out the fires, that are mainly centred on peatlands in Riau province.

Helicopters would be sent into the skies above Sumatra to inject chemicals into clouds, which prompt the formation of heavy ice crystals, and so speed up the production of rain.

But the helicopters, from the disaster management agency, would not be dispatched until Friday at the earliest as preparations first needed to be made, said Panjaitan.

About 100 firefighters tackling the blazes were finding them difficult to extinguish as they were smouldering underground in carbon-rich peatland, mostly in palm oil plantations, he said.

“It is extremely difficult to extinguish the fires which are burning under the surface of the peatland,” he said.

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He said the cloud-seeding operation would depend on weather conditions. “Hopefully there will be lots of clouds so that we can produce a lot of rain,” he added.

The worst affected area was Bengkalis district, where 650 hectares (1,600 acres) of land was ablaze, he said, adding that 555 fires had been detected in Riau up from 356 the previous month.

In Singapore, the Pollutant Standards Index soared to 172 at 3:00 pm (0700 GMT), well past the officially designated “unhealthy” threshold of 100, according to the National Environment Agency (NEA) website.

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It was Singapore’s worst haze reading since September 1997 when the number peaked at 226.

The haze problem is a recurring one which happens in the dry season as a result of forest fires in the sprawling Indonesian archipelago, some of them deliberately started to clear land for cultivation.