But an air line has been installed overnight to the cave where the group waits to be rescued.

The death of the military diver on Friday underscored the huge risks the boys face.

The diver's death brought heartache for rescuers and anxious relatives waiting outside the Tham Luang cave in the country's mountainous north — and raised serious doubts over the feasibility of attempting to bring a group of boys with no diving experience out through the cramped passageways filled with muddy water.

But rescue officials fear their options are running out given fresh monsoon rains are forecast for the coming days.

Thailand's Navy SEAL commander on Friday said rescuers may have little choice but to attempt the tricky extraction of the group, the first official admission that the 12 boys and their coach might not be able to 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.

In an update in the early hours of Saturday morning, rescue operation chief Narongsak Osottanakorn said it was "not suitable" to make the boys dive to safety yet.

Graphic / Supplied

But he indicated that further downpours might force their hand and speed up attempts to extract them despite the dangers of carrying out such a gargantuan effort.

The 12 boys, who all play in a local football team called "Wild Boars", entered the cave with their coach on June 23 but were cut off by a sudden downpour.

They were found by British cave diving specialists nine days later, dishevelled and hungry but alive, on a ledge several kilometres inside the cave.

A daunting task now awaits both the boys — aged between 11 to 16 — and their rescuers.

A round trip to the boys and back is taking some of the world's most experienced cave divers up to 11 hours to complete, through cramped passageways and fast flowing muddy waters where visibility is highly restricted.

Many of the boys are unable to swim and none have any scuba experience.

TRAGIC DEATH

The sheer danger was made all the more apparent on Friday by the death of Saman Kunan, a former Thai Navy SEAL diver, who ran out of oxygen while returning from the chamber where the boys are trapped.

He was part of a team trying to establish an air line to the chamber where the children are awaiting rescue and lay oxygen tanks along the route.

        Former Thai Navy SEAL Saman Kunan, 37, died while trying to help rescue the young football team. Photo / News Corp Australia

"We lost one man, but we still have faith to carry out our work," Navy SEAL commander Apakorn vowed.

Saman resigned from the Thai military in 2006 before working at Bangkok's Suvarnabhumi airport, according to a post on the Thai Navy SEALs Facebook page, which said he was a triathlete and a "skilled and able diver". Thai social media filled with tributes to the fallen hero.

Asked how the boys could make it out safely if an experienced diver could not, Apakorn said they would take more precautions with the children.

But experts say the dangers remain high.

Diving cylinders are prepared at a makeshift camp at the entrance of Tham Luang Nang Non caves. Photo / Getty Images

"It's very risky (diving out). 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.

The accident marks the first major setback for the gargantuan effort, which has gripped Thailand as the nation holds its breath for their safe escape.

NEW FOOTAGE TELLS BLEAK STORY

New footage of the daunting conditions facing the group trapped in the flooded cave has been captured by British television network ITV.

Several rescuers can be seen struggling through dark, narrow passages. They can see where they are going only through headlights, and are moving through the flooded cave system holding a rope above them.

It shows the enormous task ahead for the boys, several of who are weak from not eating proper food for days.