"Today is D-Day", Narongsak Osottanakorn, head of the rescue mission, told reporters before the team went in the cave.
Twelve boys, ages 11-16, and their 25-year-old coach became stranded when they went exploring in the cave after a game, reports said. 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.
Narongsak Osottanakorn, who is in charge of the rescue mission, said Saturday conditions were now "perfect", BBC News reports.
The entire operation to rescue all 13 could last two to four days, depending on weather and water conditions, said army Maj.
"Now the 13, their health and their minds are ready and they all have knowledge of the mission".
"We are giving them a physical examination".
According to our correspondent, Thai officials have warned the rescue could take three or four days in the worst case scenario.More news: Ohio State Investigators' Emails To Jim Jordan Went To Bad Address
It is unclear exactly how the boys will be moved but it appears that the journey will include some diving and walking in areas where the water level permits.
"Today we are at peak readiness", the lead rescuer said as he announced the start of the operation.
Narongsak explained the group chose to move forward with the mission because heavy rains are expected in the area in the coming days, which could have further complicated the rescue.
He added that the boys had all been assessed by a doctor and were "very fit physically and mentally". Shortly after nightfall, local time, Thai navy SEALs reported on Facebook that the four were rescued.
The preparations indicated that officials have abandoned all extraction plans except for helping the boys to swim and dive the approximately 5km from their refuge inside the cave to the outside world. Air Force Capt. Jessica Tait, a spokeswoman for U.S. Indo-Pacific Command, told ABC News at the time. The first is expected to reach safety at 9 pm on Sunday (10 am ET) at the earliest.
The letters from the boys penned by the Thai Navy SEAL who is staying with the boys in the cave.
The combination has rescuers racing against both continued monsoonal rains this weekend and oxygen levels that have dropped from 21% to 15%.