His death highlights the perils of the operation to extract the team from deep inside the waterlogged cave, raising questions about the feasibility of bringing youngsters out the same way.

"A former Seal who volunteered to help died last night about 2am," Chiang Rai deputy governor Passakorn Boonyaluck told reporters at the site, calling it "sad news."

The diver, identified as Saman Kunont, was coming back from a spot inside the Tham Luang cave where the group were located on Monday when his supplies ran short.

"On his way back he lost consciousness," said Thai Seal commander Apakorn Yookongkaew, adding that a friend had tried to help bring him out.

"But even though we have lost one man, we still have faith to carry out our work."

Asked how the boys could make it out safely if an experienced diver could not, Apakorn said they would take more precautions with children.

The accident marks the first major setback for the gargantuan effort, which started almost two weeks ago after the "Wild Boars" team went into the cave in northern Thailand after football practice.