MAE SAI, Thailand >> With time running short and storm waters threatening to rise, a team of elite divers made preparations Monday to lead a second group of boys from the depths of a flooded cave where they and their soccer teammates have been trapped for two weeks.

Four boys were brought out of the cave Sunday, according to Thai navy SEALs, who were leading the rescue operation. The boys were transferred by helicopter and ambulance to a hospital where they are being treated, but officials did not disclose details of their condition. Eight others, plus their coach, remained in the cavern.

The former governor of Chiang Rai province, Narongsak Osottanakorn, said the healthiest boys were evacuated first and the operation was going “very smoothly.”

“After 16 days of waiting … today, we saw the boys’ faces,” Narongsak said.

The operation paused Sunday evening so that rescue teams could refresh the supply of air tanks that have been placed along the escape route inside the cave, Narongsak said. The next phase of the operation was expected to begin Monday, and “the mission will continue for the remaining boys as soon as possible,” he added.

The first boy emerged from the cave at 5:40 p.m., less than eight hours after the operation began under gray skies. Officials feared that an incoming storm could send water flooding back into the cave and make an escape even more difficult for the boys, ages 11 to 16, and their 25-year-old coach.

Thirteen foreign divers and five Thai divers were leading the rescue, with each boy accompanied by two divers, officials said. A total of 90 divers, including 50 foreigners, have been involved in the entire operation, Narongsak said.

Narongsak told reporters earlier Sunday that the boys would be brought out one by one and that it could take at least 11 hours for the first person to be rescued. A Thai army commander said the entire operation could take up to four days.

But four boys had emerged by 8 p.m., quite a bit ahead of schedule — and all the more remarkable because the boys are novice swimmers, with no diving experience.

The journey was harrowing: a more than one-mile dive through narrow passageways filled with muddy water that renders visibility close to zero and flows so fast in some places that even experienced divers have had to stop or turn back.

The Thai navy SEALs gave the boys crash courses in using diving masks and breathing underwater.

The rescues were carried out with a diver holding each boy underneath him as they swam through the cave, the boy breathing through a mask and attached by a tether, while another diver swam in front of them, Narongsak said.

It usually takes divers about five hours from where the boys were to reach Chamber 3, a dry point where the SEALs have set up a command post, and from which rescue officials said the boys could probably walk the rest of the way.

Rescue teams had installed a static rope along the dive path, giving the boys a guide, and experienced divers were reportedly positioned at various points along the way for assistance. Spare air canisters had also been placed along the route.

Underscoring the danger in the fast-moving waters, a former Thai navy SEAL died on Friday while moving the canisters, reportedly because of lack of oxygen.

The high-risk rescue dive began with a sense of urgency after authorities failed to settle on an alternative means of bringing the boys out. Drilling a hole into the mountainside to lift them to safety — as 33 Chileans were rescued from a collapsed mine in 2010 — was dismissed because the boys’ location couldn’t be pinpointed accurately, and it wasn’t clear how drilling could alter the mountain’s geology.

Some officials initially said that the boys could remain where they were — on a dry rock ledge near a point inside the cave known as Pattaya Beach — for up to several months as long as they were supplied with food and medicines. But authorities became worried in recent days as oxygen levels inside the cave dropped because of the presence of a high number of rescue workers.

Teams have used high-powered pumps to empty more than 100 million of gallons of water out of the cave. On Sunday morning, Thai officials said that water levels were at their lowest point in several days, contributing to the decision to launch the operation.

The SEALs, which have stationed divers and medics with the boys for the last week, posted a message on their Facebook page on Sunday promising to bring the group back safely.

“We, the Thai team and the international team, will bring the Wild Boars home,” read the message, referring to the name of the boys’ soccer team.

The boys and their coach biked to the cave after soccer practice on June 23 and ventured deep inside the 6-mile-long cave, a popular tourist attraction but one that is mostly deserted in the summer because of the risks of monsoon rains. A storm arrived while the group was deep inside. The boys and their coach couldn’t hear the rain and were trapped when water sloshed into the cave.

They were found more than a week later by a pair of British volunteer divers who were part of a rescue mission that has drawn divers and experts from around the world — including U.S. military personnel from Japan, bird’s nest collectors who hunted for shafts in the rock face and members of Elon Musk’s SpaceX venture.

The Americans on site include an Air Force rescue support team of about 30 divers, survival specialists and medical and logistics experts.

On Sunday, as news of the first rescues flashed worldwide, President Donald Trump tweeted for the first time about the soccer team, saying the United States was working “very closely with the government of Thailand to help get all of the children out of the cave and to safety.