Narongsak Osottanakorn, the Chiang Rai governor and head of the rescue mission, said that "today, everything was smooth".
As of Monday morning, eight boys and their coach remained in the cave where they have been trapped by floods since June 23.
They spent nine days unaccounted for inside the cave, before British divers found the emaciated and dishevelled group huddling on a muddy bank.
Narongsak said Sunday two divers would escort each of the boys out of the cave. "If you ask me how long it will take, no one can answer that".
Also Monday, Prime Minister Prayut Chan-ocha visited the cave for the first time since the extraction operation began.More news: Parents of rescued boys now at hospital told 'no hugging' yet
The boys were taken straight to hospital.
Speaking a day after four boys were brought out from Luang cave, Prayuth said the operation's time frame is up to the provincial rescue command and he does not have the authority to interfere.
According to CNN, schoolchildren in Mae Sai became excited when they heard that even Elon Musk had announced that he would lend a hand to the rescue mission. One was that it was unknown how safe and dry the area where they had taken shelter would stay as Thailand's rainy season, which lasts until at least late October, picks up pace.
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". Thirty-five emergency doctors were on standby as the "exhausted" boys were flown by military helicopter or driven in ambulances to Chiang Rai Prachanukroh hospital. This was earlier than what was expected if conditions were ideal.
Narongsak said rescuers had to tighten a guide rope as part of their preparations for Monday's operation. If the rain god helps us, then we may be able to work fast.More news: Who stepped up against Utah at Summer League?
He also says the families of the boys have been informed about the risky mission.
Thai authorities have confirmed that four members of the football team that was left stranded in a cave have now been rescued. The second boy came out 10 minutes later.
He has 30 years' diving experience and was involved in the retrieval of the body of stunt diver Agnes Milowka who died at Millicent, South Australia, in 2011.
The path out is considered especially complicated because of twists and turns in narrow, flooded passageways.
The 12 boys and their coach had been trapped in the Tham Luang Nang Non for more than two weeks now.
Dozens of divers have arrived at the cave on Sunday morning.More news: Former Navy SEAL: Thailand Cave Rescue Is Like 'Climbing Mount Everest'
The operation to rescue the remaining eight boys - some as young as 11 and weak swimmers - and the coach was called off at nightfall until Monday to give the divers time to replenish oxygen supplies and ensure all preparations were complete.
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