A former navy SEAL working to rescue 12 boys trapped in a cave in northern Thailand has died from a lack of oxygen, Thai authorities have confirmed.

SEAL commander Arpakorn Yookongkaew told a news conference on Friday morning that the rescuer was working in a volunteer capacity and died during an overnight mission in which he was placing oxygen canisters.

The ABC understands the volunteer diver, Saman Kunan, 37, set out at 8.37pm (local time) to deliver three oxygen tanks from chamber three to where the boys are trapped.

SEAL commander Arpakorn Yookongkaew said on Friday he was placing the oxygen tanks in the cave in preparation for the rescue attempt and lost consciousness as he tried to return.

“We are trained to expect risks at anytime,” Commander Arpakorn said.

“It’s part of the job.”

Mr Saman was taken to hospital but died about 1am, the ABC is reporting.

With heavy rains approaching, the plan to free the boys and their soccer coach from deep within the Tham Luang cave was stepped up on Friday.

It is believed the rescuer died overnight.

Earlier, Thailand’s king urged a quick evacuation and monsoon rains threaten to raise water levels in the cave, but medical assessments of the trapped boys indicate it is still too early to attempt a rescue.

The difficult trip from the cavern where the boys are sheltering takes experienced rescue divers around five hours to complete and demands both diving and climbing skills.

A medical report released overnight warned that two of the boys and the coach were suffering exhaustion from malnutrition, while CNN quoted an unnamed navy SEAL as saying their health makes it too dangerous to attempt to move the group out on Friday.

“We have not yet decided when they will be evacuated,” Narongsak Osotanakorn, head of the rescue operations, said on Thursday night.

“We need to consider several factors including weather conditions,” Mr Narongsak was quoted as saying by Thailand’s The Nation newspaper.

While a difficult journey through the flooded cave complex remains the most likely option, authorities are not ruling out the possibility of the boys remaining in their cavern for a extended period until the monsoon rains pass and water levels drop.

Teams of searchers are also hunting the mountainside above the cave for possible alternate “chimneys” that would allow a quicker route out.

Massive pumping operations in the cave complex, coupled with a rare window in the rain, has seen water levels drop, but the boys’ health remains the deciding factor.

Planners have begun finalising the preferred plan to extract the boys, providing them with diving gear and prepositioning equipment along their evacuation route.

“We also need to consider their physical condition. Medics will decide on that. Physical rehabilitation is now going on,” Mr Narongsak said.

He said 13 sets of diving gear and escort teams have already been prepared. 

“If physical examination shows the survivors may not be 100 per cent ready, but all risks involved are within manageable levels, we will go ahead with the evacuation immediately,” Mr Narongsak said.

“Doctors and Navy SEALS will make the evaluation.”

Under the rescue plan, each of the trapped boys will be escorted by two divers on their way out of the cave.

Oxygen tanks have been positioned along the cave route. Photo: Thai Navy SEALS

Rod lines and oxygen tanks have already been installed along the route.

The boys would not carry their own oxygen tanks, instead relying on  their diver escorts for their air supply.

But as ever, the rescuers eyes are on the weather and the the possibility of an emergency evacuation.

“Our major concern is the weather because, despite our best efforts to drain the water, we cannot beat the rising floodwater level if there’s heavy rain,” Mr Narongsak said.

“The SEALS are able to go back and forth and the stranded children are at a safe location, but a higher water level would make the rescue operation even harder,” he explained. 

While the boys have been communicating with the outside world, the last footage of the group was released by the Thai Navy SEALS on Wednesday.

The video showed the boys looking emaciated after their ordeal, but in apparent good spirits.

-with agencies