The operation began at 10 a.m.
On Saturday morning, the Thai Navy posted photos of letters that the group had written to their family and the outside world.
"The divers will work with the medical team inside the cave to determine who should be brought out first".
They were joined by three more divers on Sunday afternoon and a team of rescue workers helping to run a rope system in more hard stretches of the cave, as well as five Thai navy Seals. The team and coach were exploring the cave after a practice game when heavy rainfall and flooding cut off their escape route out of the cave and prevented rescuers from finding them for nearly 10 days.
Given the risk of the oncoming rains that could undo the drainage efforts, Mr Narongsak said the conditions were now ripe for the boys to be evacuated. When they come out they want to eat many things.
Media have been asked to leave the area by 9am local time (noon, AEST) Sunday, and the site was evacuated for a "rescue operation".
The team is reportedly holed up in a chamber about 2.5 miles (4 kilometers) into the cave, with experienced divers taking about 11 hours up and back during delivery missions over the past week.
"I promise to take the very best care of the kids", he wrote. That's the name of the young boys' team. His colleagues said they would "not let the sacrifice of our friend go to waste". In summary, great news with four boys rescued and operation now on hold as oxygen supplies are renewed and the divers get some rest.
In the early hours of Saturday morning, he said the boys were not yet ready to dive out of the cave, a complex and unsafe task through twisting and jagged submerged passageways. The operation began at 10 a.m. and he said it would take at least 11 hours for the first person to be rescued.
Thailand cave rescue underway for soccer team trapped over 2 weeks
CNN's Matt Rivers said the forecast for monsoonal rains meant there might be no alternative but to try and get the boys out now. A Thai army medic sits in an ambulance on standby for the rescue operation in a restricted area near Tham Luang cave .
"If we keep on waiting and the rains come in the next three or four days, our readiness will decrease", Narongsak said, according to the AP.
Cave rescue specialists have cautioned against that approach except as a last resort, because of the dangers posed by inexperienced people using diving gear. "The boys are ready to face any challenges", rescue chief Narongsak Osottanakorn told reporters near the cave site on Sunday morning. When he checked it at 7 p.m., there were at least 20 calls from anxious parents, none of whose sons had come home.
The only way to bring them out is by navigating dark and tight passageways filled with muddy water and strong currents, as well as oxygen-depleted air.
One volunteer, an expert diver, died Thursday while laying spare air tanks along the escape route through narrow, winding flooded passages.
The Thai governor in charge of rescuing 12 boys and their soccer coach says the co-operating weather and falling water levels over the past few days have created conditions for an attempted extraction of the group.
Between the operating base manned by Thai Navy Seals inside the cave and the trapped boys are twisting, turning cave passageways with torrents of water gushing through.
Against all odds, four boys were rescued from the flooded Luang cave Sunday.
Thai officials had been suggesting in public statements that a quick underwater evacuation of the boys and their coach was needed because of the possibility that access to the cave could soon close again due to seasonal monsoon rain expected this weekend.
The boys, ages 11 to 16, and their 25-year-old coach became stranded when they went exploring in the cave after a practice game.