They are then taken to hospital in Chiang Rai.
The rescue of the first four boys had brought joy and relief to the families and friends of the 13, with one school friend of some of the boys telling media that he is very happy and thanking the rescue team for bringing his friends out.
A team of 13 worldwide cave diving experts and five Thai divers entered the precarious cave system at 10am local time on Sunday, after attempts to drill through to them were abandoned. The operation started at 10 a.m., saying that it would take hours for the first person to be rescued, according to The Associated Press.
The soccer coach trapped in a Thai cave with 12 boys for almost two weeks apologized to their parents in a handwritten note released Saturday, promising to take care of the children during the rescue mission "as best as I can".
Twelve youths aged from 11 to 16 were trapped, while a massive global rescue effort has been mounted including Thai Navy divers and foreign volunteers.
One former Navy Seal died trying to help rescue 12 boys and their coach who have been trapped in a cave for almost two weeks - highlighting the inherent risks in attempting to move the boys, who are physically weak after days without food and dwindling oxygen supplies.
A source at the Chiang Rai hospital said that five emergency response doctors were awaiting the boys and a further 30 doctors were on stand-by. Each boy and the coach are to be accompanied by two highly experienced divers.
Rescue chief Narongsak Osottanakorn told journalists near the Tham Luang cave site: "Today is the D-day".
They guide the boys through the passageways and carry their air supply.
He said the next phase of the operation will resume in between 10 and 20 hours - probably late Monday morning or afternoon - as rescuers have to replaced empty compressed air tanks and perform other maintenance on the rescue system.
Four boys were brought safely out of the cave on Sunday.
"I would like to inform the public at home and those who have been giving us support all along, after 16 days, today's the day we've been waiting for, we are seeing the Wild Boars in the flesh now", Narongsak said.
Thirteen foreign divers and five members of Thailand's elite navy SEAL unit are the main team guiding the boys to safety through narrow, submerged passageways that claimed the life of a former Thai navy diver on Friday. It was unclear who was inside the ambulance or the helicopter.
In the meantime, she says she'll continue to pray for a smooth rescue of the entire team.
Monsoon flooding cut off their escape route and prevented rescuers from finding them for nearly 10 days.
More rain is expected in the coming weeks.
"It's unsafe to the most experienced divers to go through", one diver told Reuters.
"Football's Coming Home. First Wild Boars Out", a headline on one online Thai paper said on Monday, referring to a song chanted by English soccer fans at the World Cup now underway in Russian Federation.
Cave rescue experts have said they consider an underwater escape to be a last resort, especially with people untrained in diving.