Diving teams and medical personnel entered the cave at 10am (03:00 GMT), rescue mission chief Narongsak Osottanakorn told reporters on Sunday.
"Today is D-day," he said.
Thirteen foreign divers and five Thai Navy SEAL divers are taking part in the rescue mission, which could take two to four days, officials said.
Each boy will be accompanied by two divers. It will take at least 11 hours for the first person to be rescued, meaning the first member of the team could come out at 9pm (14:00 GMT) at the earliest.
Only diving teams, medics and security forces remain at the entrance of the cave, as all non-essential staff and members of the media have been cleared from the site.
The Wild Boar football team went exploring in the Tham Luang cave network after a football game on June 23 and was trapped kilometres deep inside the cave system as floodwaters caused by heavy rains blocked the entrance.
The 12 children, aged 11 to 16, are accompanied by their coach, Ekkapol Chantawong, 25.
The group was found, dishevelled and emaciated but alive, on a muddy ledge by rescue divers on Monday. Rescue workers struggled to find a way to extract the team from the cave.
On Saturday, rescue mission chief Osottanakorn said conditions were "perfect" to evacuate the team in the coming days before fresh rains and a possible rise in carbon dioxide further imperil the group.
Rescuers have conceded that evacuating the boys is a race against time, with monsoon rains expected to undo days of around-the-clock drainage of the flooded cave.
"Now and in the next three or four days, the conditions are perfect [for evacuation] in terms of the water, the weather and the boys' health," Osottanakorn told reporters.
"We have to make a clear decision on what we can do."
Reporting from Chiang Rai, Al Jazeera's Scott Heidler said it had started raining again on Sunday.
"It's about rain falling here, but it's also about rain falling in the area," he said.
"It's a massive cave network and this is a big mountain range, so water can travel to the cave from distances."
While the oxygen level had stabilised, he warned that levels of carbon dioxide in the cave were a vital factor in considering when to move the group. In addition, the impending rains could raise the water levels to cover much of the muddy ledge on which the group are sheltering.
"The water level may rise to the area where the children are sitting and make the area less than 10sq meters," Osottanakorn said, citing estimates from cave divers and experts.
In the early hours of Saturday morning, he said the boys were not yet ready to dive out of the cave, a complex and dangerous task through twisting and jagged submerged passageways.
But his comments 12 hours later suggest the thinking had changed, with water levels inside the cave currently managed to their lowest point by constant drainage.
Industrial pumps have been working incessantly in an attempt to clear the tunnels and, hopefully, allow them to escape by foot.
After emerging from the cave, the boys will be taken straight into a waiting ambulance which will drive them to a nearby helipad. From there, they will be flown to the provincial capital of Chiang Rai for treatment in a hospital there.
In notes written by the team and published on the Thai Navy SEAL Facebook page on Saturday, the coach apologised to the parents of the boys for taking them into the cave complex during monsoon season.
"To all the parents, all the kids are still fine. I promise to take the very best care of the kids," Chantawong wrote.
"Thank you for all the moral support and I apologise to the parents."
The coach's role in the team's predicament has split Thai social media. Many have lauded him after reports emerged of him giving his share of food to the children before they were located and helping them get through nine days in darkness.
Other touching notes signed by members of the team were directed to their families, many of whom have kept an anxious vigil outside the cave for a fortnight.
"Love to Mum, Dad and my little brother," reads one note from 15-year-old Phiphat Photi - who is better known as "Nick".
"If I get out, please can you bring me some grilled pork and vegetables?"