A helicopter flew the four boys to the nearby city of Chiang Rai, where they were taken by ambulance to hospital. Eighteen divers set off at 10 a.m. local time on a unsafe mission to rescue the junior soccer team, who had been stuck deep inside Tham Luang cave for 16 days.
The next operation for the rest is set to resume in 10-20 hours, with officials and volunteers buoyed by the fact that their method of rescuing the boys by diving has been successful so far, he said.
They said rescuing the eight remaining boys and their soccer coach could take up to four days.
The risks of the journey through the cave's narrow passageways, in some places no more than 0.6m wide, was driven home last Friday when a former Thai navy diver died after his oxygen supply ran out. Narongsak said some are well enough to ask for solid food.
The president of soccer's governing body, FIFA, has invited the boys to the World Cup final in Moscow on Sunday if they make it out in time.
"The boys are ready to face any challenges", rescue chief Narongsak Osottanakorn told reporters near the cave site as weather forecasters warned of more monsoon rains late on Sunday that would cause further flooding in the cave.
On Sunday four members of the "Wild Boar" team were successfully brought out from the cave, after authorities decided they had to rush ahead with a rescue operation to beat monsoon rains.
Expert divers managed to get four of the 12 boys to safety on Sunday.
The dramatic and unsafe rescue at the Tham Luang cave in the northern province of Chiang Rai was launched on Sunday and four boys were brought out that day.
The names of the rescued boys were not released.
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"We're thankful for the coach for being there with them, to help guide the kids through this situation, without him, they probably wouldn't have survived", said Guerrero.
Thailand has waited anxiously for news of the safe return of the boys and their 25-year-old coach since they became trapped in the Tham Luang cave complex on June 23, in a saga that has dominated global headlines.
Chiang Rai acting Gov. Narongsak Osatanakorn had said earlier Monday that the second phase began at 11 a.m. and authorities "hope to hear good news in the next few hours".
In the meantime, she says she'll continue to pray for a smooth rescue of the entire team.
It is still not clear how the boys will be rescued.
The Facebook page for the Thai Navy SEALs has been posting updates on the effort.
"He added: "They can not decide how many of them will be able to come out for the first operation". After an initial assessment at the site, the plan was to airlift the boys to a makeshift helipad close to the Chiang Rai Prachanukroh hospital, some 70 km away.
Rescue divers found the 12 teenage soccer players and their coach alive on July 3 after they went missing in a Thai cave 10 days earlier.
The other, and perhaps more worrying, was that oxygen levels in the complex were falling close to risky levels.