Four more boys have been rescued from the Thai cave in a rapid-fire operation, bringing the total rescued to eight.

That leaves four young soccer players and their 25-year-old coach trapped inside the mountain in northern Thailand.

But the head of the operation would not confirm whether all five could come out today, raising questions over whether coach Ekkapol Chantawong could be left behind to spend a lonely night in the cave.

Narongsak Osottanakorn said in a press conference on Monday night that it will be up to the divers to decide whether it is possible to save all five at once. He warned that "the plan is designed for rescuing four" and "for safety, the best number is four".

The former governor of Chiang Rai province said he hoped for "100 per cent success" when the rescue resumes on Tuesday, with the mission currently on hold again for up to 20 hours.

The eight boys who have been rescued are "safe and conscious" in hospital, said Narongsak. They were placed in isolation as a precaution, although he said the authorities may soon allow the parents to see the boys.

Local officials refused to confirm the identities of those who had been saved, and their parents have not been informed of who is out. The families have agreed to remain at the cave until all of the boys and the coach are saved.

The five people still stuck in Tham Luang cave have also been assessed as healthy by a doctor, who journeyed three kilometres through the cave complex to reach them.

A senior Thai officer said the rescue plan was reviewed after Sunday's operation to make it "sharper", with Monday's evacuation completed in nine hours — around two hours faster than the previous day's operation.

Rescue teams re-entered the cave system at 11am local time on Monday (2pm AEST). The fifth boy blinked into daylight at around 4.30pm (7.30pm AEST) and was carried to a waiting ambulance on a stretcher, witnesses said. He was then transferred to a helicopter and taken to join the other rescued boys at a hospital in Chiang Rai.

The sixth boy emerged at around 6pm local time (9pm AEST), and the seventh and eighth in quick succession at 7pm (10pm AEST).

The Thai Navy SEALs confirmed on Monday night that eight of the "Wild Boar" soccer team had escaped the complex. "8TH BOAR ... Hooyah," the SEALs posted on their Facebook page.

Cheers were heard outside the cave as reports emerged that four more boys had successfully made the dangerous journey to the surface in Khun Nam Nang Non Forest Park.

But with monsoon rains pouring down again, the rescue of the final five could prove even more perilous, with many concerned rising floodwaters in the cave complex could complicate rescue efforts and affect the evacuation.

Officials insisted that storms forecast for Chiang Rai province in Thailand's far north had been factored into their decision to go ahead with the complicated and dangerous plan for the boys aged between 11 and 16 to dive out of the cave.

Those leading the effort have become increasingly upbeat, telling the media to expect "good news" ahead of Monday night's rescue despite the many challenges, and the death of a navy diver in the caves just days ago.

Narongsak said the boys were in good spirits but hungry, revealing they were craving pad kra pao — a popular dish of spicy basil chicken with rice.

The boys most up to the challenge of diving in the murky waters would be the next to be retrieved, after the weakest were reportedly saved first.

He said the water level was "not worrisome" even though heavy rains have fallen on the Tham Luang Nang Non cave system, and hoped the second phase would be "quicker" than the first operation due to lower water levels. He was proved right.

THE FIRST RESCUE

The 12 young players from Moo Pa (Wild Boar) Academy soccer squad and their coach — a former monk — became trapped on a dank ledge in the dark chamber on June 23.

It was nine days before they were found deep in the mountain by British cave specialists, some suffering from exhaustion and malnourishment.

The divers took food and supplies to the group to build up their strength, but the arrival of monsoon rains made it essential to immediately extract them.

In a race against time amid a heavy downpour, experts concluded their original plan to swim the boys out was the best option.

Thirteen divers entered the cave at 10am Sunday local time (1pm AEST) — some heading straight for the trapped group and others taking up stations along the 3km system of flooded chambers.

Ten rescuers headed to the boys in chamber nine, and to the junction at chamber six, while the others went to support positions shortly afterwards.

Each boy was to wear a full scuba mask, wetsuit, boots and a helmet as they were accompanied by two divers through the cave.

They were to be strapped to a "buddy", the leading diver, by a tether and dragged along.

This diver was to carry two tanks and share oxygen with the boy as the other followed them through the cold, murky water and airless chambers that have already claimed one life.

The foreign divers and five Thai divers entered the caves after an Australian doctor gave the all clear. Locals were required because none of the foreigners speak Thai and communication before and during the dive was key.

The second leg of the risky operation was placed on hold after Sunday's rescue as air canisters were replenished along the underwater route to where the boys and their coach have been trapped for almost two weeks.

Defibrillators were also put in place in case the rescue should take a bad turn.

Foreign Minister Julie Bishop said lessons from the initial effort would be applied as two more groups of four were brought out of the cave.

"The fact that it took so many hours underscores how precarious this whole mission is," she said.

Contrary to initial reports, it's now believed the weakest boys were selected to come out first, following an assessment by Adelaide cave diver and anaesthetist Dr Richard Harris.

Additional rescue personnel, including divers from Thailand, the US, Australia, China and Europe, were stationed between the third chamber and the entrance, where the boys would have to use a rope to traverse challenging terrain.

WHAT WILL HAPPEN NEXT?

How quickly the remaining boys are moved out will depend on the conditions and water levels inside the cave, which are likely to rise with the rain.

The governor said officials were "still at war with water and time", after experts warned him that rain could shrink the dry ledge where the boys are sheltering to just 10 sqm.

"All the plans must not have any holes in them," he said, noting that "hundreds of people have vetted this" and "there will always be margins for error".

He said floodwaters had been drained as much as possible, but the rain could increase the risk. "The plan that I've held on to from the beginning is that we have to bring the kids out and the determining factor of this plan is to have as little water as possible."

He also warned of higher carbon dioxide levels in the cave.

Water levels inside the cave are fluctuating, making it difficult to know for sure how long some of the dives will take.

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

Saman was part of a team trying to establish an airline to the chamber where the children were awaiting rescue. He had placed oxygen tanks along the route but didn't have enough air to get back to safety.

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