The spokesman said conditions must be stable, as today, for the operation to resume.
The rescue teams now needed at least 10 hours to prepare for their next operation, involving about 90 divers in total, 50 of them from foreign countries, he said. "They are now at the field hospital near the cave", said Tossathep Boonthong, chief of Chiang Rai's health department reportedly told Reuters.
"During the peak of the rainy season, we are anxious that the area where the boys are can be inundated or can be left at just 10 sq m".
The operation to bring them out of the cave, where they have been trapped for two weeks, has begun and the first of the group are expected to be freed from the cave at around 12am AEST.
The ambulances were seen Sunday evening, but it was unclear who was inside them.
The rescue started at 10 am Thai time and involved 13 worldwide divers and five Thai navy seals, who were described by Governor Narongsak as "an all-star team", reports the Daily Mail.
The boys and their coach hiked into the Tham Luang cave complex when it was relatively dry, only to be walled in after monsoon rains triggered a flash flood. The rising water filled large stretches of the cave, cutting off the team's escape route and keeping rescuers from finding the team for 10 days, CBS News noted.
"Because of the complexity of the cave and difficulty of the operation.it is unknown how long it will take before the team can bring out the first batch of boys", the rescue's Joint Command Centre said in a statement.
The head of the rescue mission, Narongsak Osottanakorn, suggested earlier in the weekend that the boys were not ready to make the trip, but later on Saturday said there was a window of opportunity to try ahead of looming monsoon rains.
"He added: "They can not decide how many of them will be able to come out for the first operation". I promise I will care for the kids as best as possible.
Authorities said efforts to drill down from the top of the mountain have been unsuccessful so far, with only 18 workable openings.
Expert divers worked together to rescue guide the boys out through the narrow passageways of the cave. General Kongcheep Tantrawanit said, referring to the area where rescue workers had set up a base.
"We estimate that (they) are 600 metres down, but we don't know the (exact) target", he said. They are physically and mentally strong, he said.
They were followed shortly afterwards by four others, leading to an explosion of jubilation on social media in Thailand and around the world as the rescued boys were rushed to hospital.
For the boys, the most risky part is the first kilometer, in which the are required to pass through a submerged channel no wider than a person.
Mr Narongsak said the evacuation decision was made after consultations with all concerned, including Seal members, the medical staff and families of the trapped persons.
"If we keep on waiting and the rains come in the next three or four days, our readiness will decrease", Narongsak said, according to the AP.
Divers are guiding the 12 boys and their coach through darkness and submerged passageways towards the mouth of the Tham Luang cave system.
"Don't blame yourself", one of the parents of the 12 young soccer players who have been trapped in a cave in northern Thailand for two weeks has written to the boys' coach.
Thai cave rescue: What happens next? Rescuers will be using air tanks and face masks while guiding the boys to safety. He says the earliest they will come out is 9 p.m. Sunday (10 a.m. ET).
Of the 13-strong foreign dive team - mainly from Europe - three were escorting the children, while the remainder were positioned along the unsafe first kilometre stretch, where the boys will have to navigate through submerged passageways in some places no more than two feet (0.6 metre) wide.
The plight of the stricken boys, who have remained trapped inside the caves for 15 days, has transfixed Thailand, as rescue efforts have become ever more urgent.
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