"When we're in a confined space, if the oxygen drops to 12 percent, the human body starts to slow down and people can fall unconscious", Osottanakorn said. They expect the first rescue to be completed by 9 p.m. Sunday.
Rescuers have to dive through flooded tunnels for six hours to reach the team and it takes another five hours to return to the entrance, CNN reported.
But with another storm brewing, he said it had to happen now.
The football coach who took the boys into the cave has written to their parents apologising for his actions.
According to Maj. Gen. Chalongchai Chaiyakam, the 13 "will continuously come out in approximately 2-4 days, which all may change depending on weather and water conditions". Each will be accompanied by two divers.
Each of the boys will be paired up with two trained divers, and it will take at least 11 hours for the first person to be brought out.
Noting that a means to free the boys has not been decided, Mr Narongsak said that the water levels from the cave entrance to chamber three - where the rescue base is - is now low enough for personnel to walk through.
Narongsak says the rescue mission was launched because floodwaters inside the cave are at their most optimal level.
"We can no longer wait for all conditions (to be ready) because circumstances are pressuring us", Thai SEAL commander Arpakorn Yookongkaew told a news conference.
He also wrote to his own family members - an aunt and his grandmother - asking them not to worry too much about him.
Thai divers gather before they enter to the Tham Luang cave, where 12 boys and their football coach are trapped, in the northern province of Chiang Rai, Thailand, July 6, 2018.
It came as authorities admitted that time is running out to get the boys out of the caves, where they have been stuck for two weeks since June 23. At this time though, diving is the only possible method of escape, even though cave rescue experts warn it is extremely unsafe even for those with experience.
Their plight has transfixed Thailand and the rest of the world, as authorities struggled to devise a plan to get the boys and their coach out through twisting, narrow and jagged passageways that in some places are completely flooded.
"Now and in the next three or four days, the conditions are flawless (for evacuation) in terms of the water, the weather and the boys' health".
Rescuers at a cave where 12 boys and their coach are trapped in northern Thailand are carefully monitoring weather maps for signs of heavy rain. As well as ambulances that have gathered at the site, American divers have arrived and the nearby relatives' room is empty.
Officials say that they have a window of up to four days before rains will make the rescue more hard.