Much of the focus on Friday will be on feeding an oxygen tube in to improve the air supply in the cave, from the third chamber and on to the fourth chamber.

"It was unusual situation and anything could happen," Yookongkaew said.

“The operations are still going on,” and the effort to rescue the children as soon as possible would continue.

Sergeant Saman Guana was undertaking an operation to refill air tanks at the time he died.

He was swimming from chamber four and approaching chamber three when he lost consciousness.

A dive buddy tried to administer first aid in the water and then got him through to the base at chamber three, where further attempts were made to revive him but it was too late.

His body was then taken to a local hospital.

The Thai King has said that the sergeant will have a royal funeral and that he will take care of the family and the funeral.

About 1.5 kilometres of the cave is still full of water, according to authorities, who also say that efforts are under way to bring more oxygen into the sprawling complex.

A Thai military army general said at a morning press conference still under way that “time is now the most important factor” in the rescue effort.

“Today the most important thing is to get the air tube into the cave,” he said.

A phone line into the cave will be run down the air tube, too, he said.

Chiang Rai province Governor Narongsak Osotthanakorn has said it is too dangerous for the Thai soccer team to be extracted from the cave for now, while also confirming fresh oxygen will be pumped in.

In comments made overnight at the Tham Luang cave complex, Mr Narongsak said conditions were good in the first three chambers of the cave and it was still possible to walk in this section.

But he warned "the problem is beyond the T-junction, where the water level is quite high. At that point, the passage bends and the water level is very high".

"According to the plan, if we let them dive out, the most dangerous point is extremely narrow, the boys will have to dive by themselves, the water is quite deep and long at that point and the [Thai Navy] SEALs can't be next to them."

"We did an assessment and we think it is quite dangerous at this point to bring them out. At this point it is too dangerous to dive."

Oxygen bottles arrived late on Thursday evening at the site, too, and the Governor confirmed that the cave's air supply was also a concern for rescue workers.

He confirmed that air in the caves had thinned because of the large number of rescue workers and now needed replenishment.

"There are two ways to solve this, for individuals to carry in their own supplies, or to fill the cave with oxygen," he said.

Loading

"Right now we are preparing a five-kilometre line for oxygen, to fill the cave with oxygen."

If a swimming and diving rescue were to go ahead, the Governor said, the best time for the boys to emerge would be at night because they had now been in darkness for 13 days. Three of the trapped members of the soccer team were suffering from exhaustion, he added.

He also confirmed that a communications cable had been laid only as far as the T-junction, so the boys had not yet been able to speak directly to their parents.

Water levels in the cave have been dropping by about one centimetre per hour and at least 130 million litres had been pumped out by Thursday.