CHIANG RAI, Thailand — The death of a former Thai navy SEAL early Friday during the rescue operation to save a boys' soccer team trapped in a flooded cave was the latest setback for a mission fraught with danger as officials raced against worsening weather.

Saman Kunan, 38, fell unconscious underwater and died around 1 a.m. local time as he attempted to return from placing air tanks deep inside the underground complex. A fellow diver tried to revive him but was unsuccessful.

"The conditions in the cave are tough," Adm. Apakorn Yuukongkaew, commander of the SEALs unit, told reporters on Friday.

"Once he placed the oxygen tanks he became unconscious on his way back. His buddy tried to administer first aid, when there was no response he tried to move him," Apakorn added. "We won't let his life be in vain. We will carry on."

Authorities have raised concerns about how much air the 12 stranded boys and their coach have access to inside the cave. The supply has been depleted by the presence of hundreds of rescue workers.

Deputy Cmdr. Chalongchai Chaiyakham of the Thai army said Friday it was unclear how long the remaining air would last.

Chiang Rai Gov. Narongsak Osottanakorn said on Thursday that rescuers were preparing a three-mile "oxygen pipeline" as part of preparations for the group's extraction but added that the boys would not be coming out soon.

"You see we are increasing the number of people going inside the cave. So we have to fill it up with oxygen," Narongsak told reporters.

Rescuers, including international teams, are considering alternative ways to bring the group out before heavy rains hit the country's north next week which could further hamper the operation.

An honor guard holds up a picture of Samarn Kunan, 38, a diver who died working to save 12 boys and their soccer coach inside a flooded cave, as family members mourn on Friday. Panumas Sanguanwong / Reuters

Rescue alternatives include teaching the boys to dive and then swim out, a highly risky venture; remaining in the cave for months until the wet season ends and flood waters recede; or drilling a shaft into the cave from the forest above.

The boys, aged 11 to 16, and their assistant coach were found inside the cave on Monday, after nine days underground, hungry but in good spirits. They went missing after they set out to explore the cave on June 23.

Rescuers are deciding how to remove the group but have been slowed by logistical issues including high water levels inside the cave and narrow, flooded passages that would require the boys to dive alone.

The Thai navy is teaching the boys the basics of diving, with a view to guiding them out through flood waters. However, some of the boys cannot swim and navigating the cave has frustrated even the most expert divers.

Volunteers at the Tham Luang cave in northern Chiang Rai province were shaken by the diver's death.

"A navy SEAL just passed away last night. How about a 12-year-old boy that will have to pass through?" Rafael Aroush, an Israeli living in Thailand and a volunteer who arrived at the cave site on Thursday, told Reuters.

"There will be rain and many things could go wrong. I don't want to say it, but it could be a catastrophe," he said.

Heavy monsoon rains are forecast for next week in most of the north, according to Thailand's meteorological department.