The operation began at around 11 a.m. local time on Monday and is expected to finish for the day around 9 p.m, reported the BBC.
The entire operation to get all 12 boys and their football coach out of the cave could take two to four days, depending on weather and water conditions, said Thai army Major General Chalongchai Chaiyakam. "We can't visit our boys in hospital because they need to be monitored for 48 hours", Somboon told Reuters.
The boys were rescued using full face masks, Narongsak said at a news conference where he described it as the "very smooth operation".
The four boys pulled from the cave Sunday in an urgent and unsafe operation that involved them diving through the cave's dark, tight and twisting passages were happy and in good health, authorities said.
Officials told Reuters that Monday's rescue operation would take between 10 and 20 hours.
Air tanks were planted along the route earlier on Monday.
As night fell, the operation to rescue the remaining eight boys - some as young as 11 and weak swimmers - and their coach was called off until Monday morning.
A short official video released by the rescue operation late on Sunday showed four ambulances with their lights flashing driving up the muddy dirt track that leads to the cave complex.
Rescue mission chief Narongsak Osottanakorn said it had resumed, with divers re-entering the cave at 11am local time (4pm NZT).
That makes for eight of the 13 people who have been trapped under increasingly unsafe conditions since late last month.
Thai cave rescue: Four more boys extracted in second operation
The second phase of a rescue operation has ended for the day, with more than half of the Thai soccer team rescued from the cave. The entire operation to rescue all 13 could last two to four days, depending on weather and water conditions, said army Maj.
The four boys who were rescued on Sunday appeared to be in good health, Thai officials said, but were undergoing a battery of medical tests in the hospital to ensure they were free from disease and infection. More than 100 personnel and 18 divers worked on Monday's mission.
The four boys rescued Sunday are recovering in a nearby hospital and are yet to see their parents.
The football team went missing on June 23, having gone into the caves after Saturday practice to do some exploring, and ended up being trapped by rising flood water. Two divers were assigned to each child to help them navigate the risky, narrow passageways. They were stranded by flash floods that caused water levels to rise, making it hard to get out. Meteorologists estimated a 60% chance of rain in the area on Monday.
On July 6, after crowdsourcing some ideas among his Twitter followers, Musk announced that he had sent SpaceX and Boring Company engineers to help the Thai government and rescue workers.
He added that the divers were the "same multinational team" that rescued the four boys on Sunday.
Rescuers found them huddled on top of a dry rock about 2 1/2 miles from the cave's entrance after nine days of searching.
In an indication of how risky the journey can be, a former Thai navy diver died in the caves on Friday.
Narongsak would not identify the rescued boys.
The rescuers are racing against time, as more rain is forecast, and the extra water would significantly complicate matters.
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