Lack of air has emerged as a new and dangerous threat to the survival of the 12 boys and their soccer coach trapped in a flooded cave complex in northern Thailand.

It was confirmed on Friday afternoon that a former Thai navy SEAL diver died from a lack of air, and there are reports that rescuers are urgently trying to pump oxygen into the cave where the 13 people remain trapped.

Saman Kunan, a former member of Thailand’s elite navy SEAL unit, had joined the rescue team in Chiang Rai as a volunteer.

He died on Thursday night after entering the cave at 8.37pm to lay air tanks along a potential exit route, the SEAL commander said.

An army general overseeing the rescue mission, Chalongchai Chaiyakam, says the cave where the boys are had only 15 per cent oxygen left on Friday.

“Our main mission today is to lay a pipe into the chamber so the group will have more air,” he said.

The pipe will have to be 4.7 kilometres long to be able to reach the chamber from the cave entrance.

Mr Saman was one of about 80 former and currently serving Thai navy SEALs involved in the mammoth rescue effort to save the boys, aged 11 to 16, and their 25-year-old coach.

On their Facebook site, the SEALS described Mr Saman, 37, as a brother and an “underwater warrior”, and vowed to push on with the rescue.

“Today, we will take a good break. We will complete this mission as well as the healing,” the post said.

ABC correspondent Anne Barker, who is at the caves, said Mr Saman’s death was a blow to the rescuers’ morale.

Saman Kunan was one of 80 divers. Photo: Facebook

“The preparations are that they would bring out the strongest boys first and it will be potentially a multiday event, depending on how many boys they bring out at one time.

“Today is the day that people are expecting their first efforts to bring these boys out,” she said.

“Whether they take this [death] as a reason not to … remains to be seen.

“Certainly, any of the methods they were assessing, including drilling from above or just waiting until the rains subside, which could be months … are all fraught with risks and difficulties,” said Ms Barker.

SEAL divers have told CNN that oxygen is being pumped into the cavern where the boys and their coach are huddled. Rescuers are also racing to pump out water from the flooded Tham Luang complex, ahead of heavy rain forecast for the weekend.

A military source told the ABC three factors were driving the urgency – the water level inside the cave, the amount of available oxygen, and the health of the boys and their coach.

“The pressure we are experiencing comes from working against the weather, as the forecasts tell us there will be heavy rain in this area within 48 hours,” he said on Friday.

Provisioning for the rescue is well underway. Photo: Facebook/Thai Navy SEALS

Efforts to drain floodwaters stepped up even further on Friday. According to News Corp, Poonsak Woongsatngiem, a rescue official with Thailand’s interior ministry, said efforts were concentrated on the cave’s third chamber.

“We [are] target[ing] 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,” Mr Woongsatngiem said.

“We can’t say whether there will be a rescue today or not. We hope there is no rain.”

After the third chamber, the boys have another 2.5 kilometres to walk out of the cave.

If walking is not possible, they will have to swim with scuba gear.

Rod lines and air tanks have already been installed along the route. The boys would not carry their own tanks, instead relying on diver escorts for their air supply.

But the group’s ill-health, after nearly two weeks underground, makes that an unattractive option.

On Thursday, medics told officials it was too risky to move them. Two of the younger boys are reportedly particularly unwell, as is coach Ekapol Chathawong, who is said to have given most of his food to the teenagers.

A diagram of where the boys are trapped, posted by Thai dive site Digitalay. Photo: Facebook/Digitalay

Above-ground, the search for an alternative route into the caves has also stepped up, amid reports the boys have told rescuers they have heard the sounds of chickens and dogs. However, parents have been told that their boys might be suffering hallucinations after so long underground.

On Thursday, Chiang Rai Provincial Governor Narongsak Osottanakorn, gave the most detailed description yet of preparations.

“I have asked for 13 sets of equipment to be prepared and to check the equipment lists and place them inside in case we have to bring them out in this condition with less than 100 per cent readiness,” he told a media briefing.

“I have asked the doctor and the SEAL team who are working on the rescue operation to assess the situation and see what risk level we can take.”

“For example, if we can accept the 90 per cent safety level, and if all conditions are at that 90 per cent, then we are ready to bring them out. And that’s what we are prepared for.”

-with agencies