Chiang Rai, Thailand (CNN) Pressure is mounting on Thai authorities to bring forward a rescue plan for 12 boys and their coach trapped deep inside a flooded cave in northern Thailand, after the death of a former navy diver and a drop in oxygen levels underground.

Officials initially thought they could keep the boys and their coach in the cave where they are trapped for up to four months, until waters dropped sufficiently for them to be able to walk out.

But the death of a rescue team member, and the realization that oxygen levels have fallen to potentially dangerous levels, appears to have forced a reassessment of the situation.

Thai Navy SEAL chief Rear Adm. Aphakorn Yoo-kongkaew said oxygen levels in the cave had dropped to 15%, a level that one Thai medic said posed a serious risk of hypoxia, the same condition that causes altitude sickness. It was too dangerous to leave the boys much longer, Yoo-kongkaew said, despite the risks involved in attempting to bring them out.

"We can no longer wait for all conditions (to be ready) because of the oppressive situation," he told journalists Friday.

"We originally thought the young boys could stay safe inside the cave for quite a long time but circumstances have changed. We have limited amount of time." He did not say how long they could survive with current oxygen levels, but he said getting more oxygen piped into the boys was top priority.

Scuba oxygen tanks being delivered to the cave rescue site on July 01, 2018.

Despite concerns, the boys are unlikely to be extracted within the next 24 hours because it was still too dangerous, according to a Thai Navy official who is not authorized to speak to the media.

One reason the team cannot yet be evacuated is that rescuers were also still searching for three more wetsuits with a thickness of five millimeters, the official, who has direct knowledge of the operation, told CNN. Three of the boys are too small for the wetsuits on hand.

But the number of rescue workers staying with the team in the chamber had been reduced from 10 people to five because of the lowered oxygen levels.

Authorities were pushing ahead with plans to free the team hours after former Sgt. Saman Kunan, a Thai ex-SEAL, died at 2 a.m. Friday (2 p.m. Thursday ET), as he returned from an operation to deliver oxygen tanks to the cave where the boys are located.

The 38-year-old ran out of air while underwater, an official said.

The boys, members of a youth football team, and their coach have been trapped in the labyrinthine cave at the Tham Luang Nang Non complex for nearly two weeks, unable to navigate their way out of a series of narrow passages after floodwaters forced them to take shelter on a rocky ledge.

The huge operation to rescue them involves dozens of Thai Navy SEALs, and experts and volunteer divers from parts of Europe and Asia, as well as Australia and the US. Billionaire inventor and entrepreneur Elon Musk said Friday engineers from his SpaceX & Boring Co. were heading to Thailand to see if they could help.

'He was a triathlete'

Former Thai Navy SEAL Saman Kunan, who died in the Tham Luang Nang Non cave complex during a massive rescue operation to save 12 boys and their soccer coach.

Kunan's death had changed the mood on the ground and made real for rescuers just how dangerous the mission has become.

"Definitely you can feel it that it has an effect, but we're moving on. Everyone is a professional so we're trying to put it away and avoid it happening again," said Finnish volunteer diver Mikko Paasi, a long-term resident of Thailand.

"Everybody is focusing on getting these boys out -- keeping them alive or getting them out."

The UK divers who first reached the boys described their dive as "gnarly" and full of tight passages submerged with opaque waters. Authorities have been considering teaching the boys to breathe through full-face oxygen masks to be pulled out. It takes even the most experienced divers up to five hours to swim through jagged, narrow channels from where the boys are to safety outside.

The death of Kunan, an experienced diver, in the cave system underlines the inherent risks in attempting to move the boys, who are physically weak after days without food.

One of Kunan's longtime friends, Sgt. Anuram Kaewchano, told CNN he was shocked to learn the news.

"I can't believe this happened," he told CNN by phone. "He was very fit, he exercised every day, and he was a triathlete. Our last trip together was to Malaysia."

He added that the last time the two spoke, "we talked about the kids -- whether they were out yet."

SEAL chief Yookongkaew said Kunan may have passed out, causing him to drown, "but we have to wait for the autopsy."

"Diving is always full of risks," he said.

A military aircraft will carry Kunan's body from Chiang Rai to Satthahip Navy Base Friday evening. A funeral service will take place there, and then at his home town in the province of Roi Et, northern Thailand.

Pornphimon Pansurin, a counselor from the nearby Darunrat School who teaches Ekarat Wongsukjan, one of the missing boys, said the missing boys may feel responsible for the tragedy.

"If they learn about what has happened they will blame themselves," she told CNN. "They will feel very guilty."

International operation

The members of the Wild Boar soccer team were reported missing on June 23 when they didn't return from an outing after soccer practice. They entered the cave during fine weather but became trapped when a sudden downpour flooded the narrow tunnels.

The 12 boys and their coach were found deep inside the cave by two British cave divers on Monday, perched on a rock slab above flood waters, after nine days without food or fresh water.

The Thai boys with Navy SEALs inside the cave on Tuesday.

Rescue teams have been pumping millions of liters of water from the cave in an attempt to lower water levels to the extent that the boys can simply walk out. However more rain is forecast this weekend, putting pressure on rescuers to act soon or have to contend with even higher water levels inside the cramped chambers.

Thailand's monsoon season runs from July to October and, while the past few days have been relatively dry, the long term forecast is rain for months.

Correction: The spelling of the deceased diver's name has been updated to reflect new information.