Thai authorities say a former Navy SEAL working to rescue the boys and their soccer coach trapped in a cave died from lack of oxygen.

Key points: Petty Officer First Class Saman Kunan was a former Navy SEAL

Petty Officer First Class Saman Kunan was a former Navy SEAL He was working in a volunteer capacity and died during an overnight mission

He was working in a volunteer capacity and died during an overnight mission He passed out while underwater and efforts to resuscitate him failed

Petty Officer First Class Saman Kunan, a former member of Thailand's elite Navy SEAL unit who was part of the rescue team in Chiang Rai, died on Thursday night after entering the cave to lay oxygen tanks along a potential exit route, the SEAL commander said.

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

The ABC understands he was diving late at night to get to the chamber where the boys are trapped.

Sorry, this video has expired Navy SEALS Commander vows diver's death 'won't be in vain'

Petty Officer Saman, 38, set out at 8:37pm local time to deliver three oxygen tanks from chamber three to where the boys and their coach are located.

He completed his task but on the way back lost consciousness. His diving partner performed CPR but was unable to revive him.

Petty Officer Saman was brought to chamber three but was pronounced dead about 1:00am (local time) on Friday.

He is one of about 80 Thai Navy SEALs. They make up the bulk of the divers that are running the rescue effort.

Saman Gunan was a former member of Thailand's elite Navy SEAL unit. ( Facebook: Poom Pui )

Rear Admiral Arpakorn said despite the tragedy, the rescuers "will continue until we accomplish our mission".

"In that 1,700-metre section [it] is very treacherous," he said.

"Last night's operation was to bring the oxygen tanks to the mound, which should take about five to six hours each way, so altogether it takes about 12 hours.

"This is what we train for, this is what we have to face, an incident like this can always happen but we're not losing our motivation.

"We won't let the life lost be wasted."

He said Thailand's King had ordered a royally-endorsed funeral ceremony.

Sorry, this video has expired Rescuers pump water and drill rock inside cave where Thai boys are trapped

Petty Officer Saman was a former clearance diver for the Navy SEALs, and Thai media has reported he was more recently working in a security role at Bangkok's Suvarnabhumi Airport.

Thai authorities are racing to pump out water from a flooded cave where 12 boys and their soccer coach have been trapped since June 23.

A navy source, speaking on condition of anonymity, told the ABC three factors were driving the urgency — the water level inside the cave, the amount of oxygen available, 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.

The soccer team was trapped in a cave system known as Tham Luang, surviving for nine days before rescue divers found them on a muddy ledge.

The navy source said two of the boys were in a weaker condition, as was the 25-year-old coach, who sacrificed his share of their meagre food supplies to give to the boys.

The source said rescue coordinators faced tough choices and there were differences of opinion among those providing assessments.

Sorry, this video has expired Expert in disaster mental health Sarb Johal says the boys' morale is heartening.

ABC, wires