A military operation in the Tham Luang Nan Nong cave complex is employing hundreds of industrial pumps to drain the water along the 4km path from the entrance to the area where the children and their football coach have been sheltering for 12 days.

Poonsak Woongsatngiem, a rescue official with Thailand’s interior ministry told the Guardian the water had been reduced by 40% in past days, clearing 1.5km stretch of dark, jagged and muddy cave channels that the boys would need to traverse.

The focus of Thursday’s operations is a third major basin along the route where water still reaches to the ceiling. The boys, who cannot swim, are being trained in how to dive using scuba equipment. But forcing them to exit through the water is fraught with risk.

“We [are] target[ting] the water in the third chamber to reduce to the point that no diving equipment is needed, like to the waistline, so one can wear just life jackets and walk out,” Woongsatngiem said on Thursday.

Clearing the third basin would leave another 2.5km to reach the boys, whose ages range from 11 to 16. It is unclear how long this remaining area would take to drain, but monsoon rains are predicted for Saturday and would quickly refill the cave complex, potentially cutting the boys off for months.

Rain was now the biggest worry, said Narongsak Osatanakorn, the governor of Chiang Rai province, who is coordinating the rescue efforts. “I’ve asked the meteorological department to check the weather forecast and report to us daily,” he said.

A fibre-optic cable was being laid inside the cave and would reach the boys soon, he added, so the children would be able to speak to and possibly see their parents on video.

With wet weather looming, authorities are also calculating how long they are willing to allow rescuers to remain in the cave before they order an evacuation – one that could cut the boys off for up to four months.

Updates on the condition of the boys and the water levels were being slowed by the length of the journey to reach the children. “It’s about 11 hours – six on the way from the entrance to where the kids are and five on the way back,” he said.

He said the boys and their 25-year-old coach had started practising diving but did not confirm whether they would try to bring any out on Thursday. Those considered strong enough to go could do so first, he said. “They don’t need to be brought out as a team,” Osatanakorn said. “Whosoever is ready can come out first.”

Other than continuing to drain the cave, officials were also scouring the jungle above the complex to find new openings that could be drilled to extract the boys more easily. The children were breathing, he said, so there was very likely a shaft somewhere to the top.

“We have around 20 to 30 teams surveying on top to find the closest and most precise spot,” he said.

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