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. He said the rescuer passed out while underwater, and efforts to resuscitate him failed.
"Despite this, we will continue until we accomplish our mission,'' Arpakorn said.
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, before more rains are forecast to hit the northern region.
On Thursday, Chiang Rai provincial Gov. Narongsak Osatanakorn said he asked the SEALs in charge of extraction plans to estimate what sort of a risk would be involved to bring out the boys and their coach even though they weren't 100 percent ready for an hourslong dive.
Officials prefer to get the boys out as soon as possible because heavy rain expected by Saturday almost surely will raise water levels again in the cave, making passage in some areas even more difficult, if not impossible. They are hoping that an upgraded draining effort can lower the water in an area where it is still at or near the ceiling. The idea is to get some headroom so the boys would not be reliant on scuba apparatus for a long stretch and could keep their heads above water.
"We can no longer wait for all conditions [to be ready] because the circumstance is pressuring us,'' Arpakorn said. "We originally thought the boys can stay safe inside the cave for quite some time, but circumstances have changed. We have limited amount of time.''
The governor has said the 13 might not be extracted at the same time, depending on their condition. They've practiced wearing diving masks and breathing, in preparation for the diving possibility.
The boys, ages 11 to 16, and their 25-year-old coach went exploring in the Tham Luang Nang Non cave in the northern province after a soccer game June 23. Monsoon flooding cut off their escape and prevented rescuers from finding them for nine days.
Cave-rescue experts have said it could be safest to simply supply the boys where they are and wait for the flooding to subside. That could take months, however, given that Thailand's rainy season typically lasts through October.
Experienced divers are wary of taking out the untrained boys through the dark and dangerous waters still in the cave.