Teams entered the cave at 10:00 local time (03:00 GMT), operation chief Narongsak Osottanakorn told reporters.
All non-essential staff have been cleared from the site near the entrance, leaving only diving teams, medics and security forces.
The boys were exploring the cave on 23 June when they were cut off by water.
Thirteen foreign divers and five from the Thai Navy are involved in the rescue mission.
The earliest the first boys could emerge is 21:00 local time (14:00 GMT) on Sunday, according to officials. It is thought the whole operation could last several days.
Both the trapped group and their families had been informed of the plan.
Mr Narongsak said that the boys and their coach were "very fit physically and mentally. They are determined and focused."
It is unclear exactly how the boys will be moved but it appears that the journey will include some diving and walking in areas where the water level permits.
The initial part of their journey, through narrow, submerged passages, is the most challenging, the BBC's Jonathan Head reports from nearby.
It could involve long periods underwater for children as young as 11 who have never used diving equipment before.
Earlier this week, a former Thai Navy diver died in the caves while returning from a mission to provide the group with oxygen.