The 12 Thai boys trapped deep inside a partially flooded cave are not yet ready to dive out, the rescue operation commander said, although forecast heavy rains could speed up their extraction attempt.

Governor Narongsak Osottanakorn, a former governor of the northern Chiang Rai province who is leading the rescue effort, said there were no plans to pull the boys and their coach out overnight but if monsoon rains fell and water rose in the Tham Luang cave over the coming days they could change their plans.

"The boys are not suitable …(they) cannot dive at this time," governor Osottanakorn told reporters. "Now the problem is the children's readiness to dive.

"We will) try to set the best plan. If the risk is minimal, we will try. We are afraid of the weather and the oxygen in the cave. We have to try to set the plan and find which plan is the best."

Thailand's Navy SEAL commander earlier said rescuers may have "limited time" to attempt the tricky job of getting the group out — the first official admission waiting out the monsoon period in the cave may not be possible.

The grim reality facing the boys and their football coach has never been more evident.

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.

Rescuers make their way down at the entrance to a cave complex where 12 boys and their soccer coach went missing. Photo / AP

With oxygen levels dropping quickly, the rescue mission seen being rehearsed in the new footage could begin at any moment.

The cave system in Mae Sai, northern Thailand, is losing oxygen as rescuers continue to try and deliver air supplies to the kids and coach through a pipe. Authorities say the ventilation pipe must reach them today.

The dwindling air supply and race against oncoming monsoonal rain have given the rescue mission fresh urgency.

The risky operation, which claimed its first fatality early Friday morning when a former Navy SEAL died from lack of oxygen, could reach its climax tonight or tomorrow as time runs out.

Explaining the difficulties of the operation on Sky News, Ivan Karadzic, a Danish cave diver volunteering in the risky mission indicated retrieval of the boys could begin at any time.

When asked how soon would it happen, Karadzic responded, "today or tomorrow, I think".

"There's few people who have experience rescuing kids from deep inside the cave," Karadzic said.

The boys smile as a Thai Navy SEAL medic help injured children inside a cave in Mae Sai, northern Thailand. Photo / AP

Rescuers this afternoon conceded their rescue window was narrowing. In the first official admission the boys cannot wait out the monsoon season underground until October, authorities have warned the window of opportunity to free them is "limited".

There is now a "limited amount of time" left and little choice but to attempt the tricky extraction and get the boys out, Thailand's Navy SEAL commander Arpakorn Yookongkaew said.

The level of oxygen in the cave where the boys are trapped has dropped to 15 per cent. The usual level is around 21 per cent.

"We can no longer wait for all conditions (to be ready) because the circumstance is pressuring us," Yookongkaew said.

"At first we thought that we could sustain the kids' lives for a long time where they are now, but now, many things have changed. We have a limited amount of time."

NEW METHOD FLOATED AS TIME RUNS OUT

As the window for safe rescue narrows, engineers are reportedly considering drilling into the cave rather than having the boys swim out with Navy SEAL divers.

Drilling was ruled out as an option early on, but Thanes Weerasin, the president of the Engineering Institute of Thailand, has revealed that a drilling crew are looking at a tunnel with a large hole about 100 metres into it, the Guardian has reported.

"It's a big hole, about 1.2m by 1m," Weerasin said.

"You can go down using a rope … I think this place can lead to the children because after your foot touches the ground below, you can walk through using the compass and direct it to the tunnel [where the children are stuck]."

Australian drilling expert Kelvin Brown, who was part of the 2010 rescue of 33 Chilean miners trapped 700m below ground, told the ABC drilling could be useful, but it would come with risks.

"We knew what the formation was, we knew if there were faulted zones, the presence of aquafer," Brown said.

"I'm not too sure all that information is actually at hand in this Thai scenario. But it's all possible, as was proven.

"It makes a bit more sense to me anyway, at least for the sake of safe access, to use a drill hole."