After several days of clear skies, rain fell several times on Friday, raising the prospect that the big gains made in pumping at least 130 million litres of water out of the cave could be reversed.

Much stronger monsoon rains have been forecast to arrive on Sunday, which could stymie the rescue attempt which has gripped Thailand and become a huge global story.

FIFA president Gianni Infantino on Friday invited the trapped "Wild Boars" soccer team and their coach to the World Cup final in Moscow this month if they are rescued in time.

The former Thai navy SEAL, Petty Officer First Class Saman Gunan, 37, died in cold waters deep inside the cave complex about 2am on Friday, local time, after he ran out of air.

Petty Officer Saman had completed an operation to deliver air tanks and was swimming from chamber four to chamber three, the main operating base for the SEALs and divers within the complex, when he lost consciousness.

A dive buddy tried to administer first aid in the water and then got Petty Officer Saman through to chamber three, where further attempts were made to revive him but it was too late. His body was then taken to a local hospital and the Thai king said that he would have a funeral with full honours.

As the 12 Thai soccer players, aged 12 to 16, and their 25-year-old coach entered their 13th day trapped in the cave, much of the focus of the rescue effort on Friday was in feeding an oxygen tube four kilometres into the cave to improve the air supply, which has worsened because of the influx of rescue workers.

Navy SEAL commander Rear Admiral Apakorn Yookongkaew told the media that staff morale was still good despite the death of Sergeant Gunan and that the stretch of cave where the former SEAL had died was "treacherous”.

"Today our main goal is to get the air tube to the students because the lack of oxygen in the cave is affecting the kids,” he said, adding a phone line into the cave will be run down the air tube, too.

In a sign of how fluid the situation is at the cave site, Admiral Yookongkaew warned: "We used to think that the students would be able to survive in the cave but now many things have changed.

"The [rescue] plan can be adapted and changed,” he said, adding that "time is now the most important factor”.

General Chalorngchai Chaiyakum, the deputy commander of the Royal Thai Third Army, told Fairfax Media on Friday that "We do worry about CO2 [levels] in the cave”.

But he could not confirm how much oxygen levels had decreased in the cave because "the team inside don't send exact information yet”.

Oxygen bottles had already arrived late on Thursday evening at the site and the governor confirmed that the cave's air supply was a concern for rescue workers.

The final 1.7 kilometres of the cave is flooded with water, according to authorities.

Representatives for American entrepreneur Elon Musk are also in talks with Thai authorities about aiding the rescue.

Musk's companies could help by trying to locate the boys' precise location using Space Exploration Technologies (Space X) or Boring Company technology, pumping water or providing heavy-duty battery packs known as Tesla Powerwalls, a spokesman said.

Danish volunteer diver Ivan Karadzic, who owns a diving school in Koh Tao, Thailand, and who has has made resupply dives as far as the T-intersection inside the caves, told Fairfax Media he would "be here until either they are either successful or they ask us to leave. Everything I own is here, we have sent all our equipment."

He believed there was no alternative but to rescue the boys and their coach by diving and swimming.

"I can't see any alternative, maybe now with what happened [the death] they will consider the alternative of staying in there. The pumping is doing something, the water level is going down, but I have no idea how many millions of litres of water they need to pump out, it is probably astonishing. Diving is possible [for the kids], but it is not safe. I don't know what they will choose to do."

Overnight on Friday, the governor said that if the boys were to 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."

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.

with AP, Bloomberg