MAE SAI, Thailand — Authorities overseeing the rescue operation for 12 boys and their soccer coach from a flooded cave in northern Thailand said they have a "limited amount of time" to get them out, as they raced against worsening weather and lessening oxygen underground.

The massive operation inside and around Tham Luang Nang Non cave suffered its first death Friday when a former Thai navy SEAL passed out underwater and could not be revived.

"We can no longer wait for all conditions (to be ready) because circumstances are pressuring us," Thai SEAL commander Arpakorn Yookongkaew told a news conference. "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."

Oxygen levels are decreasing because of the amount of workers inside the cave and workers were trying to run an oxygen line into the chambers in addition to the oxygen canisters used by divers, Chiang Rai province Gov. Narongsak Osatanakorn said late Thursday.

A senior army commander, Maj. Gen. Chalongchai Chaiyakam, said the most pressing mission is the oxygen line. It is tied to a telephone line to provide a channel of communication for the kids, who are stuck deep in the complex but are being looked after by four SEALs, including a medic.

The boys, aged 11-16, and their 25-year-old coach went exploring in the cave after a soccer game June 23. Monsoon flooding cut off their escape and prevented rescuers from finding them for almost 10 days as the only way to reach them was by navigating dark and tight passageways filled with muddy water and strong currents.

Authorities have been racing to pump out water from the cave before more storms in the coming days raise the water levels again. At this time though, diving is the only possible method of escape, even though cave rescue experts warn it is extremely dangerous even for those with experience.

Friday's death of the former SEAL underscores those risks. The diver was working in a volunteer capacity and died during an overnight mission in which he was placing oxygen canisters along the route divers use to get to the children, Arpakorn said.

The strategically placed canisters allow divers to stay underwater for longer during what is about a five-hour trip to reach the stranded team.

While underwater, the rescuer passed out and efforts to resuscitate him failed, Arpakorn said. Another navy official said he didn't believe the man's oxygen tank ran out.

"Despite this, we will continue until we accomplish our mission," Arpakorn said.

The governor has said the 13 may not be extracted at the same time, depending on their condition. They boys are weak but for the most part physically healthy. They've practiced wearing diving masks and breathing, in preparation for the diving possibility.

Officials prefer to get the boys out as soon as possible because heavy rain is expected by Saturday.

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.

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. And without proper oxygen levels, staying put could also prove deadly.