MAE SAI, Thailand (TNS) — Thai SEAL divers Sunday extracted the first members of a Thai boys' soccer team from a flooded cave where they had been trapped for two weeks as an operation began to rescue all 12 players and their coach.

Four boys were brought out of the cave, according to the SEALs, who were leading the rescue. The boys were transferred by helicopter and ambulance to a hospital where they were being treated, but officials did not disclose details of their condition.

The acting governor of Chiang Rai province, Narongsak Osatakorn, 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 oxygen tanks that have been placed along the escape route inside the cave, Narongsak said. The next phase of the operation would begin 20 hours and "the mission will continue for the remaining boys as soon as possible," he said.

The first boy emerged from the cave at 5:40 p.m., less than eight hours after the operation began. 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.

A Thai army commander said the entire operation could take up to four days.

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

The rescues were carried out with each diver holding a boy's body underneath him as they swam through the cave, the boy breathing a supply of oxygen, Narongsak said.

It usually takes divers about five hours from where the boys were to reach chamber three, a dry point where the SEALs have set up a command post, and from which rescue officials said the boys could likely 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 oxygen 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 due to lack of oxygen.

The high-risk rescue dive began with a sense of urgency after authorities could not 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 due to a high presence 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 begin the rescue.

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

They were found 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.