A former Navy seal taking part in the rescue of 12 young footballers and their coach trapped in Tham Luang cave in Chiang Rai passed out during a dive and died early on Friday morning.
Naval Special Warfare Command chief Apakorn Yukongkaew said Petty Officer 1st class Saman Gunan, 37, who was a security officer at Suvarnabhumi airport, lost consciousness as he was returning used air tanks to Chamber 3 about 1am.
His diving buddy tried to help, but in vain.
Rr Adm Apakorn stressed that the former Seal's tragic death will not undermine the effort to rescue the boys.
He said His Majesty the King had given instructions that Petty Officer 1st Class Saman be given dignified funeral rites.
Chiang Rai deputy governor Passakorn Boonyaluck also said the King expressed his condolences over the loss of Petty Officer Saman. A royal bathing rite will be held at Sattahip Naval Base in Chon Buri on Friday evening.
The King also gave instructions to that the dead diver's children be well taken care of.
The death underlines the intense risks of the mission to rescue the young footballers, but the Seal commander insisted the rescue will continue nonstop until it is complete.
The rescue teams are now pumping oxygen into the underground chamber where they have been trapped for nearly two weeks as officials warned it is still too dangerous to extract the young team through flooded caverns.
The oxygen issue has raised new fears about the safety of the boys if they cannot be extracted from the caves before predicted monsoon rains this Sunday which authorities fear could re-fill the cave and reverse gains made in the mammoth rescue effort in recent days. (continues below)
Narongsak Osoththanakorn, the commander of rescue operation, said the large number of rescue workers now in the cave had lowered oxygen levels and workers were feeding a 5km cable to where the boys have been sheltering to pump fresh air into the cavern.
Pumping continued overnight to drain water from a still-flooded section of the cave where the boys remain trapped in the hope it will recede enough to allow rescuers to float the boys out, rather than have to shepherd them through dark, narrow passages in murky water.
So far, more than 130 million litres of water have been pumped out of the cave at a rate of 180,000 litres an hour, but rescuers are still struggling to plug every water source that flows into the cave and water levels beyond the fork of the cave where the boys are trapped has been agonisingly slow to drop.
Mr Narongsak told reporters overnight that it was still too dangerous to try and shepherd the boys out underwater.