Chiang Rai Gov. Narongsak Osatanakorn said Tuesday's intricate and high-risk operation began just after 10 a.m. and involves 19 divers. A medic and three Thai navy SEALs who have stayed with the boys on a small, dry shelf deep in the flooded cave also will come out, he said.
"We expect that if there is no unusual condition ... the four boys, one coach, the doctor and three SEALs who have been with the boys since the first day will come out today," he told a news conference to loud cheering.
Narongsak said this phase might take longer than the previous two rescue missions, with the longer first mission taking 11 hours.
The eight boys brought out by divers over the previous two days are in "high spirits'' and have strong immune systems because they are soccer players, a senior health official said.
Doctors were being cautious because of the infection risk and were isolating the boys in the hospital. They did get a treat, however: bread with chocolate spread that they had requested.
The plight of the boys and their coach has riveted Thailand and much of the world -- from the heart-sinking news they were missing to the first flickering video of the huddle of anxious yet smiling boys when they were found 10 days later by a pair of British divers. They were trapped in the Tham Luan Nang Non cave that became flooded by monsoon rains while they were exploring it after a soccer practice on June 23.
Two of the boys possibly have a lung infection, but all eight are generally "healthy and smiling,'' he said.
"The kids are footballers, so they have high immune systems,'' Jedsada said. "Everyone is in high spirits and are happy to get out. But we will have a psychiatrist to evaluate them."
It could be at least seven days before they can be released from hospital, Jedsada told a news conference.
Family members have seen at least some of the boys from behind a glass isolation barrier, and Jedsada said doctors might let the boys walk around their beds on Tuesday.
It was clear that doctors were taking a cautious approach. Jedsada said they were uncertain what type of infections the boys could face, "because we have never experienced this kind of issue from a deep cave.''
If medical tests show no dangers, after another two days parents will be able to enter the isolation area dressed in sterilized clothing and staying 2 meters away from the boys, said Tosthep Bunthong, Chiang Rai Public Health chief.
The second group of four rescued on Monday are ages 12 to 14.
Four ambulances and a convoy of other vehicles arrived at the cave site Tuesday morning to prepare for the third phase of the rescue.
Heavy rains in the morning cleared during the day, a reassuring sign for rescuers who have feared monsoon rains could imperil the rescue.
Narongsak said he was grateful for Musk's support, but the equipment was impractical for the rescue mission.
Musk on Tuesday visited the cave and posted pictures and videos online. He said he left the equipment there in case rescuers could use it in the future.