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.
The US official said a plan to route oxygen into the chamber where the boys are was also called off due to challenge of routing cable through cave system.
Although three of those stranded had been described as weakening, Osottanakorn said the team was in relatively good condition overall.
"The kids are very determined and they are of high-spirit".
"I hope he is still healthy and would come out soon".
Teams are drilling multiple shafts through hundreds of meters of mountainside to try to reach them while industrial pumps are working round the clock in an attempt to clear water from the tunnels and hopefully allow them to escape on foot.
While the oxygen level had stabilised he warned levels of carbon dioxide in the cave were another factor in considering when to move the group, in addition to impending rains, which could cover much of the muddy ledge on which the group are sheltering.
The group became trapped deep inside the cave after flood waters gushed in.
It could involve long periods underwater for children who have never used diving equipment before.
"As you can see, all the officials have a mountain on their shoulders", Narongsak said.
The diver was found about half a mile from where the 12 young soccer players and their coach have been trapped since June 23.
After a short deluge of rain on Saturday night and with more bad weather forecast, Narongsak on Sunday said authorities had to act immediately. Authorities said Saturday that oxygen had fallen to about 15% - normal safe levels are around 21%, while a drop to 12% could have an effect on brain function.
The rain appears to be forcing the hand of emergency officials considering a hard and unsafe effort to rescue the youngsters.
"We are trying to rule out the impossible", he said. "We don't want to make a mistake".
He also says the families of the boys have been informed about the risky mission.
"At 9pm last night we started clearing many things because we have reached the peak point of readiness", he said.
A former Thai Navy SEAL diver was killed during the rescue mission.
"And I promise I will take care of the kids as best as I can".
ABC News reports a team of four elite divers who will lead the rescue operation, and they might be brought out in stages. The boys wrote with both homesickness and humour.
It would be 11 hours before the first person is rescued.
The group sent their notes after receiving letters from their families, in their first communication since attempts to establish a phone line inside the cave failed earlier this week. If I can go out, bring to me pork BBQ please. "I love you Dad, Mom".
It's believed that two of the boys, as well as the 25-year-old coach, who were in a weak condition when discovered, have improved considerably since then. The parents had written to him earlier, assuring him that they did not blame him for the situation. "I want to say thanks for all the support and I want to apologize to the parents". "I deeply apologize to the parents".