A former Thai navy diver has died while taking part in efforts to rescue 12 boys and their football coach trapped in a flooded cave in Thailand.
Petty Officer Saman Gunan lost consciousness on his way out of the Tham Luang cave complex, where he had been delivering supplies.
"His job was to deliver oxygen. He did not have enough on his way back," said an official.
He was brought out by his dive buddy but could not be revived.
PO Saman Gunan had left the navy but returned to help the rescue operation.
Said to be an avid runner and cyclist, he was part of a massive rescue operation launched after the group found themselves stranded in the Tham Luang cave by flooding.
"Inside the cave is tough," Thai Seal commander Rear Adm Arpakorn Yookongkaew told reporters. "On the way back from setting up oxygen bottles, Petty Officer First Class Gunan passed out."
"His buddy tried to give him first aid, but he did not respond. We brought him to chamber three and gave him another round of first aid, but he remained unconscious. So we took him to the hospital."
But he said the search operation would go on.
"I can guarantee that we will not panic, we will not stop our mission, we will not let the sacrifice of our friend go to waste."
Around 1,000 people are involved in the rescue operations, including navy divers, military personnel and civilian volunteers.
PO Gunan's death has underscored the dangers behind the search efforts.
Rear Adm Arpakorn said the team still had "faith" to carry out their work.
When asked how the group could make it out safely if an experienced diver could not, he said they would take more precautions with the children, who are aged between 11 and 16, and their 25-year-old coach.
Authorities now say there are concerns about falling oxygen levels in the chamber where the boys and their coach are trapped.
Oxygen levels were being depleted by the large number of people working inside the cave network, said Chiang Rai Governor Narongsak Osotthanakorn.
Authorities are now working to get a 5km (3 mile) cable into the cave to supply the group with air.
The group were finally reached by two British rescue divers late on Monday, nine days after they entered the caves. They had been trapped by rising water while exploring.
They are reported to be in good health, and are now being regularly supplied with food and medical care.
Authorities are still trying to work out how best to bring them all to safety, with officials stressing they do not intend to take any risks with the boys' safety.
Rescue operations have for days been pumping water out of the cave, trying to get levels to drop.
If the water does not go down, the children will have to learn how to use diving equipment or wait months until the rainy season ends.
With the water inside the cave being fed by sinkholes and stream in the hills above, there is though also a chance the cave chamber they are now living in could flood completely.
"At first, we thought the children could stay for a long time... but now things have changed, we have a limited time," Rear Adm Arpakorn warned.