That left four boys and the team's coach trapped inside as the operation was called off for the day.
"The mission has resumed", a government official closely involved in the mission told Reuters, declining to be identified. Thai Navy SEALs and worldwide cave diving experts brought out four boys Sunday, and the operation is continuing today.
Authorities have been rushing to extract the boys, ages 11-16, and their coach from the cave as the annual monsoon bears down on the mountainous region in far northern Chiang Rai province.
Four of the boys were safely rescued on Sunday after rescuers carried out a risky operation, amid fears the waters will rise again to a unsafe level.
Rescue operations on Monday were expected to end by 9 p.m. local time.
The first four boys, from the group of 13, including the coach, were rescued on Sunday. The Associated Press reports that four ambulances were seen leaving the area near the cave entrance on Monday, with patients taken to waiting helicopters.
The boys brought out of the dark, winding cave on Monday are "safe and conscious", said Chiang Rai province acting Gov. Narongsak Osatanakorn, who is in charge of the rescue effort.
But Narongsak said earlier that mild weather and falling water levels over the last few days had created optimal conditions for an underwater evacuation that won't last if it rains again.
Officials said the boys freed Sunday were hungry but in good health, and had asked for holy basil stir-fried rice, a popular Thai dish.
There was no word on the condition of any of the people brought out on Monday.
Prime Minister Prayuth Chan-ocha went to the cave to inspect the rescue operation later on Monday, with navy rescuers giving him a rousing cheer.More news: Sen. Doug Jones vows 'independent review' of Trump's Supreme Court nominee
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The ordeal has riveted Thailand and captured the world's attention.
The rescued boys were being treated by medics at the field hospital, and were later airlifted to a larger nearby hospital.
The freed boys are yet to meet their parents, who late Sunday night were yet to be told which of their sons had been removed, according to the UK's Guardian.
"On behalf of the United States, congratulations to the Thai Navy SEALs and all on the successful rescue of the 12 boys and their coach from the treacherous cave in Thailand".
The hordes of global and local media have been kept back from the cave and the hospital in Chiang Rai, where the rescued boys are believed to be under observation.
Thai officials had said Sunday that they were temporarily suspending the rescue effort so they could replenish oxygen supplies and other gear.
Medical teams previously said concerns included hypothermia and an airborne lung infection known as "cave disease", which is caused by bat and bird droppings.
The trapped boys have been divided into groups.
"We have not been told which child has been brought out".
After his visit, he went to Chiangrai Prachanukroh Hospital to visit the rescued boys. I'm happy being here inside, the navy SEALS have taken good care. They are still quarantined away from their parents, who have been able to see them through a glass window but not touch or hug them. It was an incredible result considering that the boys spent nine days incommunicado, without food, until they were found, and then waited days more before embarking on an hours-long dive that even the most skilled cave divers have described as among the most risky they have attempted.