Tech billionaire Elon Musk has arrived in Thailand and gifted the country a submarine named after the football team who have spent more than two weeks stuck in an underground cave.

Musk yesterday tweeted he had been testing a kid-sized submarine which he was sending to Thailand to help with the rescue operation to free the boys and their coach.

Four more boys were rescued overnight, bringing the total number brought out of the Tham Luang Nang Non cave system to eight.

The critical section of the rescue mission is a choke point where a sharp upward bend is followed by an equally sharp downward slope. Picture: 9NEWS (Nine)

Thai authorities have suspended the rescue operation allowing the team of divers to recover and prepare for the next phase of the rescue.

Divers now face a desperate race against heavy rain to rescue the remaining four boys and their coach with further falls expected over the coming hours.

Musk today tweeted he had just returned from cave 3 and left the submarine which has been made out of rocket parts - should rescuers need it for the next stage of the operation or in future.

Tech billionaire Elon Musk has visited the cave site in Thailand. Picture: AAP (AAP)

It remains unclear if the five remaining members of the Wild Boars team will be released today or if divers will stick to their schedule of bringing out four per day.

9NEWS reporter Ben Avery, who is on the ground in Thailand, said the second stage of the operation took just 5.5 hours.

"That compares to the 11 hour round trips that these divers were making earlier in the week just to go and visit the boys and to check on their welfare," he said suggesting divers are getting used to the conditions.

A helicopter takes one of the rescued boys to safety. (AP/AAP)

The Wild Boars team and their coach have been stuck in a cave for more than two weeks. (Supplied)

The boys are said to be in good health despite their ordeal. (Nine)

He the boys were taken to the local hospital where they joined the four who were rescued on Sunday.

“We are told they are also recovering very well,” he said.

“They are in good health. They have been eating, they have been eating diluted porridge, they should be able to eat more fuller meals in the coming days.”

Highly trained divers were working in tough conditions inside the cave. (CNN)

The boys’ parents are also at the hospital but are understood to be having only limited contact with their parents due to a quarantine period being in place.

Their families were being kept at a distance because of fears of infection and the emaciated-looking boys were eating a rice-based porridge because they were still too weak to take regular food, authorities have said.

Thai Prime Minister Prayut Chan-o-cha visited the hospital overnight after speaking with the families, who are all still gathered near the cave entrance.

Former Chiang Rai governor and rescue mission commander Narongsak Osotthanakorn said that those rescued in the latest phase were in better condition than those freed the previous day.

Dramatic rescue

Thai authorities are remaining tight-lipped on the names of those rescued out of respect for the families who still have loved ones trapped inside the cave.

In the latest rescue mission, the first boy was seen on a stretcher just before 4:30 pm local time and was taken by helicopter and ambulance to the hospital in Chiang Rai.

A stretcher carrying what is believed to be a fifth boy rescued from a cave in northern Thailand is carried from an ambulance. (AAP) (AAP)

The fifth boy is loaded in to the air ambulance. Picture: AP (aap)

Two more boys left the Tham Luang Nang Non cave complex a short time later and were transferred to a medical facility on site, followed soon after by a fourth boy, according to an eyewitness working with the rescue team and stationed at the entrance to the cave.

All were wearing full face dive masks and dive suits as they were carried out of the cave to a makeshift medical centre nearby.

They were later transferred to hospital by helicopter and ambulance, with support teams using large umbrellas to screen the boys from media and other onlookers.

Thai Prime Minister Prayut Chan-o-cha inspects the rescue plans ahead of the latest operation. Picture: AP (AP)

The Thai PM talks to military personnel involved in the rescue mission. Picture: AP (aap)

Danish cave diver Ivan Karadzic was stationed at "Camp 6" inside the cave system, changing the oxygen tanks of the divers as they emerged. He told CNN that the boys were wearing several wetsuits to "minimise heat loss," which is a concern due to their "very skinny" bodies and the cold water.

The divers involved in the latest operation said it was "even more smoothly executed than yesterday," according to Mr Karadzic.

He added that divers would likely be bringing new oxygen tanks into the cave overnight and putting them at "strategic places" along the route. He described this as a necessary contingency measure due to the length of the route being traversed by the divers and children.

Thai Prime Minister Prayut Chan-o-cha posies for a group photo with family members of a group of football players and their coach who have been trapped in a cave for two weeks. Picture: AP (AP)

Boys face new challenges



Speaking to Today 's Ben Fordham, Brisbane based Rescue expert Dr Bill Griggs said the boys are facing physical, emotional and psychological health issues.

"The physical health, there is the risk they may have picked up infections or may have picked up contaminant from drinking water in the cave," he said.

"Emotionally, there is a whole stack of issues about them having been together and now being separated and trying to get them back together again and psychologically, it is obviously a very significant event and will continue to be in their lives."

Dr Griggs said doctors would work to make sure the boys were clear of infection after so long underground.

"But there is also this emotional and psychological element," Dr Griggs said.

"So one of the things is that there is eight of them out now, but there is still five inside. So that group of 13 will have bonded very strongly and so they are all waiting, the ones on the outside, waiting for the ones on the inside to come out."

Flooded passageways still dangerous

Officials warned it might take "days" to bring all 12 boys and their coach to the surface. Each boy is being accompanied by two divers and it takes hours to negotiate the flooded tunnels through the dark, murky water.

Those still inside the cave are perched on a small muddy ledge, surrounded by floodwater and with a limited supply of oxygen.

Heavy rains are approaching the Thailand caves where four boys and their soccer coach remain trapped. (AP)

Divers had little room to move in parts with parts of the cave flooded. (9news)

The most dangerous part of the journey out of the labyrinth cave system is the first kilometre, during which they are required to squeeze through a narrow flooded channel.

Rescuers need to hold the boys' oxygen tanks in front of them and swim pencil-like through submerged holes. Having completed this section, the boys are then handed over to separate, specialist rescue teams, who help assist them through the remainder of the cave, much of which they can wade through.

Divers are racing to beat the next downpour, which could further complicate efforts to remove the boys and their coach.

What's next?

The rescue workers, who are now resting, need at least 20 hours to prepare for the third operation, Mr Osotthanakorn said, but timings could change depending on weather and water levels.

Yesterday's rescue was carried out four to five hours ahead of schedule due to favorable conditions, he said.

Friends and family wait for updates on the rescued boys' conditions. Picture: AP (aap)

Officials confirmed the operation had been paused to allow oxygen tanks depleted during the first phase to be refilled.

The 12 boys, aged between 11 and 16 and all members of the youth soccer team known as the Wild Boars, entered the caves with their coach on June 23 and became trapped inside after heavy rains flooded the entrance.

They were found by two British divers last Monday, sparking a week of fevered activity around the cave system as teams of local and international experts devised a series of rescue plans.