Medical teams have been rehearsing for three days and are ready to treat the group when they emerge, officials said.
The site around the entrance to the cave was cleared of media and other people earlier today, sparking specualtion a rescue attempt was likely to begin.
"Assessing the situation now, it is necessary to evacuate the area for the rescue operation," The Guardian reports Mae Sai police commander Komsan Sa-ardluan saying over a loudspeaker at the site.
"Those unrelated to the rescue operation, please evacuate the area immediately."
There has been speculation mounting for some hours that the rescue operation is imminent, if not already under way. A press conference was scheduled for about 2pm Sunday, NZ time, but had not begun as of 2.30pm. It is expected officials will then confirm divers have not yet entered the cave system, but will soon.
As the large media contingent began packing up equipment, ambulances arrived at the mouth of the limestone cave.
"From the situation assessment, we need to use the area to help victims," police said.
There has been a flurry of activity at the site today. As well as ambulances that have gathered at the site, American divers have arrived and the nearby relatives' room is empty.
"Officials also announced that during the operation no one is allowed to take photos and if any unsuitable photos come out the person will be held responsible," The Guardian reports.
Journalists on the scene report that "the trend toward an imminent rescue attempt continues", news.com.au reports.
CNN's Matt Rivers said the forecast for monsoonal rains meant there might be no alternative but to try and get the boys out now. The situation has become more urgent over the last 24 hours due to oxygen levels dropping inside the cave. While rain has held off within the area, forecasts are calling for more and more rain.
Rivers noted green netting and barricades had been erected outside the cave entrance and that "it feels different here", despite confirming no official word had yet been made.
"Journalists are now being forced to leave the area and about a dozen additional ambulances have been brought in. This rescue attempt appears extremely imminent. No official word yet," he tweeted.
Worried that heavy monsoon rain could soon make the job even more difficult, Thai officials said earlier on Saturday they may need to quickly rescue 12 boys and their soccer coach from a partially flooded cave by helping them make risky dives to safety.
The boys, ages 11-16, and their 25-year-old coach have been trapped for two weeks — since June 23, when they went exploring in northern Thailand's Tham Luang Nang Non cave after a practice game.
Monsoon flooding cut off their escape and prevented rescuers from finding them for almost 10 days.
The only way to reach them was by navigating dark and tight passageways filled with muddy water and strong currents, as well as oxygen-depleted air.
Getting out via the same route looks like the only feasible option, but a high-risk one, Thai officials say. Experienced cave rescue experts consider an underwater escape a last resort, especially with people untrained in diving, as the boys are.
The path out is considered especially complicated because of twists and turns in narrow flooded passages.
The local governor supervising the rescue mission said Saturday that mild weather and falling water levels over the last few days had created appropriate conditions for an underwater evacuation, but that they won't last if it rains again.
Thai officials are stressing that they may have to act very soon — meaning within the next couple of days. If weather forecasts are correct, access to the cave could soon close again due to flooding from seasonal monsoon rains. Earlier efforts to pump out water from the cave have been set back every time there has been a heavy downpour.
Chiang Rai acting Gov. Narongsak Osatanakorn said authorities were waiting for two big groups of volunteer foreign divers to arrive this weekend, after which they will be ready to act quickly to bring the team members out when the conditions are right.
Narongsak said experts told him flooding from new rain could shrink the unflooded space where the boys are sheltering to just 10 square meters (108 square feet).
"I confirm that we are at war with water and time from the first day up to today," he said. "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 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: "Mom 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, Mom, 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.
"Night loves Dad and Mom and brother, don't worry about me. Night loves you all," wrote Night, in the Thai manner of referring to one's self in the third person.
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, of indistinct origin, asked their teacher not to give them a lot of homework.
In a letter of his own, the coach, Ekapol Chanthawong, apologized 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 apologize to the parents," he wrote.
An update Saturday from the Thai navy said three navy SEALs were with the boys and their coach, one a doctor. The 13 are having health evaluations and rehabilitation, and are being taught diving skills. Food, electrolyte drinks, drinking water, medicine and oxygen canisters have been delivered to them. A major concern of the rescuers is that oxygen levels in their safe space could fall dangerously low.
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.
The death on Friday of a former Thai navy SEAL, Saman Gunan, underscored the risks of making the underwater journey. The diver, the first fatality of the rescue effort, was working in a volunteer capacity and died on a mission to place oxygen canisters along the route to where the boys and others are sheltered.
Rescuers are also pursuing other options to extract the boys, hoping that finding a shaft or drilling into the mountain in which the cave is located will lead them to a sort of backdoor entrance.
Tech billionaire Elon Musk has sent a team of engineers to Thailand to see if they can help in the rescue effort. Musk's Boring Company digs tunnels for advanced transport systems and has advanced ground-penetrating radar.
A spokeswoman for the Boring Company who declined to be named said it is in talks with the Thai government and people on the ground to determine how they could best assist their efforts.
- AP
