A Thai diver drowned on Friday while helping to rescue a young football team trapped inside a flooded cave, as officials warned the window of opportunity to free the youngsters was "limited".
The diver's death raises serious doubts over the safety of attempting to bring the 12 boys and their football coach out through cramped passageways deep inside the waterlogged Tham Luang cave.
But Thailand's Navy Seal commander indicated that rescuers may have little choice but to attempt the tricky extraction of the group, in the first official admission that they cannot wait out the monsoon underground.
"At first, we thought the children could stay for a long time... but now things have changed, we have a limited time," Apakorn Yookongkaew told reporters on Friday.
A sombre mood has clouded the elation from earlier in the week, when the boys were found alive on a ledge kilometres inside the cave.
The diver who drowned, identified as Saman Kunan, passed out while returning from the chamber where the boys are trapped.
The former military diver was part of a team trying to establish an oxygen line to the chamber where the children are awaiting rescue.
Many of the boys - aged between 11 to 16 - are unable to swim and none have diving experience.
Chiang Rai deputy governor Passakorn Boonyaluck delivered the "sad news" of the diver's death to reporters massed at the entrance to the cave complex.
"On his way back he lost consciousness," Navy Seal commander Apakorn said, adding he died despite desperate efforts to help bring him out.
"We lost one man, but we still have faith to carry out our work."
Asked how the boys could make it out safely if an experienced diver could not, Mr Apakorn said they would take more precautions with the children.
Even for expert divers the journey is an exhausting 11-hour round trip.
"It's very risky. Think about it, a Navy Seal just passed away last night, so how about a 12-year-old kid," said Rafael Aroush, an Israeli volunteer helping the rescue bid.
Officials vowed to investigate the death, and have sent his body to a local hospital for an autopsy.
The accident marks the first major setback for the gargantuan effort, which started almost two weeks ago after the "Wild Boars" team went into the cave in northern Thailand following football practice.
Their ordeal has gripped Thailand, with the nation holding its breath for their safe escape.
Messages of support and prayers for the boys have flooded social media.
But the unprecedented rescue mission has been dogged by the threat of rains.
On Friday morning forecast downpours held off, giving a glimmer of respite to the rescue workers who are pumping water out of the cave while plotting the complicated extraction plan.
The team have been receiving basic training in breathing and managing diving equipment in case the order to evacuate is given.
But authorities are reluctant to allow them to dive out of the cave, given the extreme risks.
Other options under consideration are waiting out the monsoon - which could take months - or climbing out through a crack in the rock face, if one can be found.