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And the biggest challenge is yet to come. The four boys who swam out on Sunday were selected because they were the strongest of the team. It is unclear how weaker and more vulnerable members of the group will manage the journey.

"If we wait and the rain comes in the next few days we will be tired again from pumping and our readiness would drop. If that's the case, then we have to reassess the situation," said Narongsak Osottanakorn, the former Chaing Rai governor who is overseeing the rescue operation.

He added that the operation could only resume once air tanks had been replenished and other systems reinstalled.

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Sunday's operation will be remembered as an extraordinary achievement in a drama that has left Thailand and the world holding its breath.

The Wild Boars football team and their coach went missing after heavy rains blocked the exit of the cave complex, which they were exploring after football practice, on June 23.

They were found sheltering on a ledge four kilometres inside the cave on July 2 by Rick Stanton, a former West Midlands fire fighter, and John Volanthen, an IT consultant from Brighton, who are considered among the best cave diver rescuers in the world.

Authorities considered several rescue strategies, including drilling an escape shaft from above or pumping out enough water to enable them to walk out. Diving out was always considered the most dangerous option.

A family member prays near the Tham Luang cave complex, where 12 schoolboys and their football coach are trapped credit: Reuters

A Thai Navy Seal involved in the rescue died after running out of oxygen in a submerged section of the tunnel last week, and even elite cave divers have described the conditions inside the cave as frightening.

But after weather forecasters warned that fresh monsoon rains were imminent, rescuers realised they had no choice but to act quickly or risk seeing the boys drown.

"Today is the D-day. The boys are ready to face any challenges," Mr Narongsak announced earlier on Sunday morning. “A new storm is coming. If we wait and rain water comes in, our readiness will be lower than now.

Thirteen international divers set off to reach the 12 boys and their coach at 9am on Sunday morning. They were supported by 90-strong team of Thai Navy Seals and elite divers from countries including Britain, Australia, the US and China.

They then escorted four of the boys, who have been given rudimentary diving training, on a route that required them to swim for a full kilometre underwater.

The boys were equipped with "positive pressure" full-face diving masks designed to prevent water leaking in and clung to a guide rope to make sure they did not panic or lose their way in water so dark it has been likened to "cold coffee".

The team had to squeeze through gaps just 38 centimeters wide, where the rescue divers were forced to remove their air tanks and push the equipment ahead of them.

Thai media named the first boy out as 13-year-old Mongkol Boonpia. However, his mother, who has been camping by the cave entrance, said she had not been informed by authorities whether her son was among the four rescued on Sunday. She planned to stay at the cave site overnight.

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At least two of the rescued boys were helicoptered to the Chiang Rai Prachanukroh Hospital, 37 miles away. A third was being treated at a Navy Seal field hospital at the cave entrance.

While little has been revealed about the medical condition of the survivors, their ordeal is likely to take both a psychological and physical toll. Medical staff involved in the mission said they prioritise checking on the boys' breathing and signs of hypothermia. But they could also be looking for deadly diseases known to afflict miners and underground explorers.

"Cave disease", an airborne lung infection caused by bat and bird droppings, can be fatal if it is untreated. Each boy will have a devoted medical unit consisting of at least one doctor, two nurses, a paramedic and an ambulance.

It is understood that 13 helicopters - one for each boy - have been placed on standby to fly the survivors to hospital.

01:58AM Rescue expected to resume soon

Reporters at the site say activity there suggests the evacuation operation is going to restart soon.

11:10PM How it is being done

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This shows just what a difficult task the divers and the boys face in getting out the cave system.

11:06PM Rescue could take four days

Officials say it could take up to four days to complete the rescue of eight boys and their soccer coach from inside a northern Thailand cave, according to the latest details reported by the Associated Press.

Rescuers said their efforts on Sunday had gone as well as possible. But the operation had to be suspended overnight as teams replenished air tanks.

Navigating miles of twisting, flooded tunnels is difficult and dangerous. At the same time, rescuers are working flat out to ensure they have enough air to make the journey backwards and forwards to the ledge where the remaining eight boys and their coach are trapped.

10:41PM Quarantine tents

Military and police personnel at the the quarantine tent in Tham Luang cave area where the rescued boys are checked credit: Chiang Rai Public Relations Office/AFP

When the boys emerge from the cave they are brought to these quarantine tents for a medical check-up. Staff said they would be checking for healthy breathing and signs of hypothermia, as well as symptoms of "Cave disease" - an airborne lung infection caused by bat and bird droppings.

08:37PM Meet the Wild Boars team

The Wild Boars football players who went missing

Here's the list of players and their coach who went missing:

Ekarat Wongsukchan , 14 (nickname: Bew) – plays goalkeeper and promised his mother he would help at her shop after he was rescued

, 14 (nickname: Bew) – plays goalkeeper and promised his mother he would help at her shop after he was rescued Pipat Pho , 15 (Nick) – is not a member of the Wild Boars but joined their practice on June 23 to spend time with his friend Bew

, 15 (Nick) – is not a member of the Wild Boars but joined their practice on June 23 to spend time with his friend Bew Pornchai Kamluang , 16 (Tee) – plays defence and told his parents not to worry in his letter home. “I'm very happy," he wrote

, 16 (Tee) – plays defence and told his parents not to worry in his letter home. “I'm very happy," he wrote Panumas Sangdee , 13 (Mig) – another defender, and wrote to his parents that the Thai Navy Seals were taking good care of him

, 13 (Mig) – another defender, and wrote to his parents that the Thai Navy Seals were taking good care of him Adul Sam-on , 14 – a midfielder, who spoke to the rescuer when they were first discovered. He is in a volleyball team that came second in a North Thailand-wide tournament

, 14 – a midfielder, who spoke to the rescuer when they were first discovered. He is in a volleyball team that came second in a North Thailand-wide tournament Prajak Sutham , 15 (Note) – plays in midfield, although sometimes fills in in goal, and is described as a "smart, quiet guy" by his family

, 15 (Note) – plays in midfield, although sometimes fills in in goal, and is described as a "smart, quiet guy" by his family Peerapat Sompiangjai , 17 (Night) – right winger. It was his birthday the boys were celebrating when they went missing and his parents have told him they are waiting to hold his party

, 17 (Night) – right winger. It was his birthday the boys were celebrating when they went missing and his parents have told him they are waiting to hold his party Sompong Jaiwong , 13 (Pong) – was seen in an England shirt but apparently dreams of playing for the Thai national team, according to his teacher

, 13 (Pong) – was seen in an England shirt but apparently dreams of playing for the Thai national team, according to his teacher Duganpet Promtep , 13 (Dom) – striker and captain of the Wild Boars, and has reportedly been invited for trials by several Thai professional clubs

, 13 (Dom) – striker and captain of the Wild Boars, and has reportedly been invited for trials by several Thai professional clubs Chanin Vibulrungruang , 11 (Titan) – the team’s youngest player has been playing football since the age of seven

, 11 (Titan) – the team’s youngest player has been playing football since the age of seven Nattawut Takamrong , 14 (Tern) - told his parents not to worry about him

, 14 (Tern) - told his parents not to worry about him Mongkol Booneiam , 13 (Mark) – is a trainee with the team and described by his teacher as a "very respectful and good child"

, 13 (Mark) – is a trainee with the team and described by his teacher as a "very respectful and good child" Ekapol Chantawong (Ake), 25 - the a ssistant coach apologised in his letter to the parents for leading the boys into the cave , but they said they did not blame him

07:55PM Elon Musk's mini-submarine

With the dive rescue under way again in the morning, it seems as if Elon Musk's offer of help is not needed. However, he has just tweeted a video of his mini-sub being tested.

07:40PM Rescued boys are undergoing checks in hospital

An ambulance reportedly carrying some of the rescued boys leaves the Tham Luang cave area for hospital credit: Tang Chhin Sothy/AFP

The rescued boys were taken to the hospital in the town of Chiang Rai, the provincial capital, for evaluation.

06:50PM Elon Musk's company is at the site

The California tunnel company run by Elon Musk is continuing to maintain a presence at the cave. Sam Teller, spokesman for Boring Co, said that the company has four engineers who are "offering support in any way the government deems useful."

Mr Musk tweeted early on Saturday that he was working with a team from his Space X rocket company to build a "tiny kid-size submarine" to transport the children.

Mr Musk says the sub would be light enough to be carried by two divers and small enough to get through narrow cave gaps.

06:38PM Buddy system

Details are emerging of how the boys were helped out.

Each of the four boys - wearing a full face mask - was accompanied by two divers. They were tethered to a diver in front, who also carried their air tank. A second diver brought up the rear.

06:01PM So far, so good

"Today we managed to rescue and send back four children to Chiang Rai Prachanukrua Hospital safely," the head of the rescue operation, Narongsak Osottanakorn, told a news conference.

"... It's a big success of all teams. We have thousands of people helping us with the operation."

Thirteen foreign divers and five members of Thailand's elite navy Seal unit guided the boys to safety through narrow, submerged passageways that claimed the life of a former Thai navy diver on Friday.

05:57PM Donald Trump comments on rescue

An American military official told CNN that 36 Pacific Command personnel were involved in the operation, including 12 search-and-rescue divers, as well as staff from the US embassy.

03:23PM Onlookers watching the evacuation

We hear the rain is intensifying...

Onlookers watch and cheer as ambulances deliver boys rescued from a cave in northern Thailand to hospital in Chiang Rai credit: Lauren DeCicca/Getty Images

03:20PM Thai official heading the cave rescue operation says the healthiest have been taken out first.

Earlier we understood that rescuers had prioritised getting the weakest boys out first.

03:11PM Rescue paused for at least 10 hours

If only four have been rescued, it means there are still nine people - eight boys plus coach Ekkapol Chantawong - are still in the cave. It will be a nail-biting night.

03:08PM Rescuers have 'run out of oxygen'

The elite diving rescue team that brought four children to the surface has now used up its oxygen tanks and needs ten hours to prepare for the next stage of the operation, authorities have revealed.

"Now we used up all the oxygen tanks and we need to redo the whole thing," Chiang Rai Governor Narongsak Osottanakornaid said at a press conference.

"We have to make sure all factors are at the best readiness before we can start the next operation. We need to evaluate all teams and will have meeting with the whole team at 9 PM for further steps."

He revealed that the four boys who had been rescued had to swim a full kilometre to freedom.

Ninety divers, 40 of them Thai and 50 of them foreign, are involved in the operation

The rescue itself was carried out by a team of 13 international expert divers and ten Thai Navy SEALs.

Four boys have been brought out of the cave today. Earlier the Thai defence ministry said six had been rescued.

The first boy emerged at 17:40 local time (11:40 GMT) - much quicker than originally anticipated.

02:56PM Rescued children to be checked for 'cave disease'

Medics will check the rescued children for a deadly infection that is known to affect people caught in underground chambers, Reuters reports.

'Cave disease' is a lung infection spread by bat and bird droppings and can be fatal if left untreated.

They will also be checked for hypothermia and be given a psychological assessment.

Little has been revealed about the boys' physical condition, but reports that the survivors have walked out suggest they are still fairly strong.

02:30PM 'Mixed bag' of emotions amid confusion over numbers rescued

The Telegraph's Nuttakarn Sumon reports from the scene:

"A helicopter just flew over the press centre to cheering from the crowd here.

"It's a very confusing situation and a real mixed bag of emotions here. The Thai Navy Seals Facebook page says a fourth member of the Wild Boar football team left the cave at 19:47 local time (13:47 GMT).

"We understand that two have been taken to hospital by helicopter and a third is being treated at a SEAL field hospital near the cave entrance.

"But that contradicts the earlier reports that six have been rescued.

"There is a lot of rumour and conjecture here. We don't know if any of the boys are in critical condition or not."

02:15PM British volunteer divers are leading the rescue

02:11PM But the mood remains tense...

02:08PM Rescuers allowing themselves to smile...

Rescue workers along the main road leading to Tham Luang Nang Non cave credit: Linh Pham/Getty Images

02:06PM 'Weakest first'

Thai authorities have confirmed a few more details about the operation so far:

The rescuers chose to bring out the weakest boys first.

Each of them will have been escorted out underwater by two experienced divers.

There is so far little information about the boys' health, but all of the six rescued so far have been strong enough to walk out of the cave themselves rather than being carried on stretchers.

They have been given immediate medical attention at a field hospital before being taken by ambulance and helicopter to the hospital at Chiang Rai, about 37 miles away.

01:56PM Children transferred to Chiang Rai hospital

We understand that the boys are being ferried by ambulance and then helicopter to the hospital in Chaing Rai. Several ambulances have been seen leaving he cave entrance and driving towards a helipad.

An ambulance believed to be carrying rescued schoolboys travels to a military helipad credit: SOE ZEYA TUN/ REUTERS

01:51PM Six boys out of cave

Thai authorities are confirming that six boys have been brought out of the cave. As we noted earlier, that leaves seven people - six more boys, plus their football coach - waiting to leave.

The operation seems to be going much more quickly than initially expected. We originally were told the whole operation could take days. We're hours into it and half the trapped boys are already out.

01:45PM Two more boys expected out 'shortly'

That would make six brought to safety by our count. Six more boys plus their coach remain inside the cave.

One of two ambulances seen leaving the cave in northern Thailand after the rescue operation began credit: Sakchai Lalit/AP

01:39PM Military helicopters believed to be carrying rescued boys seen leaving the area

A military helicopter believed to be carrying rescued schoolboys takes off near Tham Luang cave complex in the northern province of Chiang Ra

01:35PM Thirteen helicopters on standby

Slightly contradictory reports about the progress of the rescue. Thailand's defence ministry says that four boys have reached the cave system's chamber three, where they have been offered treatment before walking the rest of the way out.

Earlier we heard that at least two had reached the surface and - later - had been helicoptered to hospital.

What we do know is that the elaborate rescue effort appears to have been successful so far.

There are 13 helicopters on standby - one for each member of the trapped team - waiting to take them to hospital.

01:29PM Helicopters evacuating boys to hospital

One helicopter with two boys took off from here at 19:10 (13:10 GMT) and will be landing in the hospital in town soon, Nuttakarn Sumon reports from the scene.

01:26PM Fourth boy rescued

Four boys among a group of 13 trapped in a flooded Thai cave reached the rescue base camp inside the complex on Sunday and will walk out soon, the country's defence ministry spokesman told AFP.

"Four boys have reached chamber three and will walk out of the cave shortly," Lieutenant-General Kongcheep Tantrawanit said, referring to the area where rescue workers had set up a base.

01:26PM Grueling underwater route to freedom

A reminder of the challenges facing the boys and their rescuers.

The ledge the boys have sheltered on is four kilometers inside the cave system.

The route out involves swimming underwater for 15 minutes at a time, squeezing through 38 cm gaps so narrow that divers will have to remove their kit, and scaling a five meter cliff.

It would be challenging in any circumstances - and diving out was considered a last resort until forecasts of rain made it clear there was only a few days left to rescue them.

01:15PM Ambulance seen leaving

Two ambulances seen leaving cave in northern Thailand hours after operation began to rescue trapped youth soccer players.

We know that three of the boys are out of danger.

The boys have had to dive, swim, and clamber through four kilometers of cave tunnels, much of it flooded, to reach safety.

01:09PM Third boy is 'safe'

A third member of the football team has reached safety and is expected to emerge from the cave shortly, it is being reported.

That leave nine boys, plus their coach, still awaiting rescue.

01:01PM Confirmed - two rescued from Thai caves

The first two members of a Thai schoolboy soccer team have been rescued from the flooded cave where they had been trapped for more than two weeks, a local rescue official said on Sunday.

Authorities in northern Chiang Rai province began the dangerous mission to bring out the 12 boys and their coach earlier on Sunday.

"Two kids are out. They are currently at the field hospital near the cave," said Tossathep Boonthong, chief of Chiang Rai's health department and part of the rescue team.

"We are giving them a physical examination. They have not been moved to Chiang Rai hospital yet,"

12:59PM 'Two boys rescued' - Thai official

Local Thai officials have said two boys have been brought to the surface. The claim has yet to be confirmed.

Thirteen foreign divers and five members of Thailand's elite navy SEAL unit launched an operation to bring the boys - some as young as 11 and weak swimmers – to the surface this morning.

They will have to swim through through narrow, submerged passageways that claimed the life of a former Thai navy diver on Friday.

06:36AM Brothers in arms

The Thai Navy Seals have posted an image on Facebook after the rescue operation was announced, showing rescue workers holding each others' arms and emphasizing the work with international rescuers.

06:13AM Whole operation could take 2-4 days

A Thai army commander says the ongoing rescue could take two-four days depending on conditions inside the partially flooded cave.

According to Maj. Gen. Chalongchai Chaiyakam, the 13 "will continuously come out in approximately two to four days, which all may change depending on weather and water conditions."

05:26AM Rescuers had to act immediately

After a short deluge of rain on Saturday night and with more bad weather forecast later on Sunday, Narongsak said authorities had to act immediately.

"There is no other day that we are more ready than today," he said. "Otherwise we will lose the opportunity."

Sustained heavy rains could make the water rise to the shelf where the children were sitting, reducing the area to "less than 10 square meters", Narongsak had said on Saturday.

Ambulances wait outside the Tham Luang cave complex credit: Reuters

05:09AM 13 foreigners taking part in rescue

A total of 13 foreigners and five Thai divers are taking part in the rescue.

Rescue chief Narongsak Osottanakorn said the boys will gradually come out accompanied by two divers each.

The only way to bring them out of Tham Luang Nang Non in Chiang Rai province is by navigating dark and tight passageways filled with muddy water and strong currents, as well as oxygen-depleted air.

Rescuers arrive nearthe cave where 12 boys and their football coach have been trapped since June 23 credit: AP

In a sign of the challenges facing the divers and boys, a former Thai navy SEAL passed out making the dive on Friday and died.

Experienced cave rescue experts consider an underwater escape a last resort, especially with people untrained in diving, as the boys are.

The rescue chief earlier that mild weather and falling water levels over the last few days had created optimal conditions for an underwater evacuation that won't last if it rains again.

04:46AM Extraction will take about 11 hours

The divers went in at 10am local time - about 45 minutes ago - and the 13 people in the cave have been informed of the operation, as have the families.

The earliest the first group will emerge from the cave is 9pm, rescue chief Narongsak Osottanakorn told reporters near the cave site.

"The boys are ready to face any challenges," he said.

04:39AM Rescue mission under way

Rescue efforts for boys in Thai cave have begun, the mission chief has told a press conference.