The boys, aged 11 to 16, and their assistant coach were found inside the cave on Monday after nine days underground.
It is likely to take up to 8 hours to get each boy the 1.7 kilometres, by swimming and diving, from deep inside the cave to the third chamber.
Four of the 12 Thai schoolboys were rescued from a flooded cave on July 8 as divers launched a daring and unsafe mission to free the children and their soccer coach who have been trapped underground for more than two weeks, officials said. The rescuers and boys will have to contend with navigating tight passageways filled with muddy water and strong currents, as well as with oxygen-depleted air.
Rescue workers inside the cave in northern Thailand preparing ahead of the first rescue operation.
Rescuers have been unable to extend a hose pumping oxygen all the way to where the boys are, but have brought them some oxygen tanks.
On Saturday, one father told Sky News he felt confident his son was "in safe hands" after receiving a letter from the boy, brought to parents by rescuers. It depends on the water levels.
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The rescue mission is proving to be hard for divers whose efforts are continually hampered by rising water that has filled sections of the cave, often forcing them to stop.
Earlier in the day, Tossathep Boonthong, chief of Chiang Rai's health department, told Reuters that the first two kids had been taken to a field hospital near the cave. Video by Kate Geraghty.
As of Sunday, four boys were confirmed rescued from the cave, before the mission was paused, according to the BBC.
"This is the most scary situation that a person could go through, said Anmar Mirza". The families of the youths had been informed of the plan, he said.
President Donald Trump is praising the rescue efforts underway in Thailand to save a boys soccer team trapped in a cave and touted the help the United States is giving. He's an anaesthetist from Adelaide. The rescue efforts were put on hold overnight Sunday as rain started "bucketing down" over the cave.
The Thai government released a graphic showing how the rescue from the Tham Luang cave would occur.
Sunday's mission actually went quicker than it had in rehearsals, Osotthanakorn said, which took place over the last three to four days.
"After 16 days of waiting, we finally get to see the faces of the "Wild Boars" today", Narongsak said, referring to the name of the boys' football team.