The operation has begun to rescue 12 boys and their football coach who will need to dive out of the flooded Thai cave where they have been trapped for more than two weeks.
The acting Chiang Rai governor told reporters “today is D-Day” with 13 foreign and five Thai divers taking part in the rescue.
He said the earliest the first person to could be rescued is 3pm BST.
Thai army commander Major General Chalongchai Chaiyakam said the 13 “will continuously come out in approximately 2-4 days, which all may change depending on weather and water conditions.”
- ITV News Correspondent Debi Edward is at rescue site
The only way to bring them out of Tham Luang Nang Non in Chiang Rai province is by navigating dark and tight passageways filled with muddy water and strong currents, as well as oxygen-depleted air.
A former Thai navy SEAL passed out making the dive on Friday and died.
Experienced cave rescue experts consider an underwater escape a last resort, especially with people untrained in diving, as the boys are.
- Soldiers, divers, underwater medics and navy SEALs arriving at the rescue site
The path out is considered especially complicated because of twists and turns in narrow flooded passages.
But the governor supervising the mission said earlier that mild weather and falling water levels over the last few days had created optimal conditions for an underwater evacuation that will not last if it rains again.
Before announcing that the rescue was under way, authorities ordered the throngs of media that have gathered at the cave from around the world to leave.
The boys, ages 11-16, and their 25-year-old coach became stranded when they went exploring in the cave after a practice game June 23.
Monsoon flooding cut off their escape and prevented rescuers from finding them for almost 10 days.
Authorities had said that incoming monsoon rains that could send water levels in the cave rising, coupled with falling oxygen levels in the enclosed space, added to the urgency of getting those trapped out.
Earlier efforts to pump out water from the cave have been set back every time there has been a heavy downpour.
Mr Narongsak said on Saturday that experts told him water from new rain could shrink the unflooded space where the boys are sheltering to just 10 sq m (108 sq ft).
“I confirm that we are at war with water and time from the first day up to today,” he said on Saturday.
“Finding the boys doesn’t mean we’ve finished our mission. It is only a small battle we’ve won, but the war has not ended. The war ends when we win all three battles — the battles to search, rescue and send them home.”
The boys sounded calm and reassuring in handwritten notes to their families that were made public on Saturday.
The notes were sent out with divers who made an 11-hour, back-and-forth journey to act as postmen.
One of the boys, identified as Tun, wrote: “Mum and Dad, please don’t worry, I am fine. I’ve told Yod to get ready to take me out for fried chicken. With love.”
“Don’t be worried, I miss everyone. Grandpa, Uncle, Mum, Dad and siblings, I love you all. I’m happy being here inside, the navy Seals have taken good care. Love you all,” wrote Mick.
The most touching note came from one whose name was not clear: “I’m doing fine, but the air is a little cold, but don’t worry. Although, don’t forget to set up my birthday party.”
Another asked their teacher not to give them a lot of homework.
In a letter of his own, the coach, Ekapol Chanthawong, apologised to the boys’ parents for the ordeal.
“To the parents of all the kids, right now the kids are all fine, the crew are taking good care. I promise I will care for the kids as best as possible. I want to say thanks for all the support and I want to apologise to the parents,” he wrote.