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.

In this July 3, 2018, image taken from video provided by the Royal Thai Navy Facebook Page, Thai boys smile as Thai Navy SEAL medic help injured children inside a cave in Mae Sai, northern Thailand.  The Thai soccer teammates stranded more than a week in the partly flooded cave said they were healthy on a video released Wednesday, as heavy rains forecast for later this week could complicate plans to safely extract them. (Royal Thai Navy Facebook Page via AP)
New video shows Thai boys in cave saying they're healthy
01:52 - Source: CNN

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.

Related: What are the options for rescuing trapped team?

‘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.

Kun