The rescuer, a former Thai SEAL, was working in a volunteer capacity and died during an overnight mission in which he was placing oxygen canisters, Thai SEAL commander Arpakorn Yookongkaew told a news conference.
Chiang Rai acting Gov. Narongsak Osatanakorn said authorities were waiting for two big groups of volunteer foreign divers to arrive this weekend, after which they will be ready to act quickly to bring the team members out when the conditions are right.
"The kids are still able to walk around, play around comfortably".
Divers took the boys letters written by family members camped outside the cave but an attempt to send down a phone failed earlier this week.
"Once his mission was over he dove back, but in the middle of their return his buddy found Samarn unconsious in the water and tried to pump his heart, but he could not save his life", the SEAL unit said in a statement. Players from teams including England and Sweden have recorded good luck video messages for the trapped boys, and Federation Internationale de Football Association has invited the 13 to the World Cup final should they be rescued in time and are healthy enough to travel.
The letter, signed by Fifa President Gianni Infantino, also said they would like to invite the youngsters and their coach to the World Cup final in Russian Federation - if they are rescued in time and well enough to travel.
Narongsak said experts, who themselves have made the hard journey, were teaching them the rudiments of diving, but that the boys were not yet capable of an arduous and complicated mission.
However, Ivan Katadzic, a Danish diving instructor who has been ferrying oxygen tanks inside the cave, said after a dive on Friday evening he was "double positive" about the mission because the water level had dropped considerably. "We have to try to set a plan and find which plan is the best". Authorities have said the soccer players are being looked after by Thai navy SEALs, including medics, staying with them inside the cave.
A military aircraft will carry Kunan's body from Chiang Rai to Satthahip Navy Base Friday evening. "I will have to use the word "try" here because I can not make a commitment that something like this won't happen again. Our last trip together was to Malaysia".
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I felt like I was dealing with it really, really nicely, and it gave me great confidence for this year coming back on grass. The Swiss, who beat Marin Cilic in last year's final, will play either Lukas Lacko or Benjamin Bonzi in the second round.
"Definitely, you can feel it that it has an effect, but we're moving on".
He said that the last time the two spoke, "we talked about the kids - whether they were out yet". "Everyone is a professional so we're trying to put it away and avoid it happening again", he said, adding: "Everybody is focusing on getting these boys out - keeping them alive or getting them out".
Elon Musk, the entrepreneur behind the Tesla automobile and the SpaceX rocket company, said he would send engineers to help.
Messages of support for the "Wild Boars" team have come in from across the world, including from football stars in Russian Federation for the World Cup.
The group became trapped deep inside the cave after flood waters gushed in.
Rescuers have been slowed by logistical issues including high water levels inside the cave and narrow, flooded passages which would require the boys to dive alone.
Thailand's monsoon season runs from July to October and, while the past few days have been relatively dry, the long term forecast is rain for months.
The boys and their coach have been trapped by flooding since June 23 and were found by divers Monday.
CNN's Kocha Olarn reported from Chiang Rai, Euan McKirdy reported from Hong Kong while Lauren Said-Moorhouse reported from London.