Chiang Rai provincial acting Gov. Narongsak Osatanakorn said the four healthiest boys were taken from the cave first. The rescued children have been taken to the hospital, though their condition was not immediately clear.
Narongsak, who noted that the operation is going "better than expected," added that the next phase of the operation would start in 10-20 hours.
The boys and their coach had been stranded in Tham Luang Nang Non when they went exploring in the cave after a scrimmage June 23. Monsoon flooding cut off their escape and prevented rescuers from finding them for almost 10 days.
Narongsak, who is heading the operation, said earlier Sunday that 13 foreign and five Thai divers were taking part in the rescue, and two divers will accompany each boy as they are gradually extracted.
The entire operation to get all 13 out of the cave could last two to four days, depending on weather and water conditions, army Maj. Gen. Chalongchai Chaiyakam said.
The only way to bring them out of the cave is by navigating dark and tight passageways filled with muddy water and strong currents, as well as oxygen-depleted air. A former Thai navy SEAL passed out making the dive Friday and died.
Mild weather and falling water levels over the past few days had created optimal conditions for an underwater evacuation that won't last if it rains again, Narongsak had said.
The Associated Press contributed to this report.