Sura Jeetwatee, a doctor who joined the rescue operation, told Reuters the boys were able to walk and had survived by staying where they were and drinking water dripping from stalactites.

"They were smart to drink water in the cave," the doctor said.

Chiang Rai Governor Narongsak Osatanakorn told The Nation newspaper the health of the boys and their coach had been checked using a field assessment coding system in which red represents a critical condition, yellow serious and green stable.

"We found most of the boys were in a green condition," he said. "Maybe some of them have injuries which would be classified as a yellow condition. But no one is in a red condition."

The governor held a news briefing, telling soldiers, monks, government officials, journalists and volunteers at the site, "They are all safe."

Rescuers shook hands and congratulated each other as occasional cheers broke out. Relatives waiting at a shelter were seen cheering, smiling and receiving calls.