The group is trapped 4km from the cave mouth
The missing Thai footballers had gone into the cave as part of an initiation-type event, one of the divers who helped find them has said.
The boys, aged between 11 and 16, are believed to have been inside the Tham Luang Nang Non cave complex to write their names on the walls before coming back out.
But a flash flood trapped them inside the complex along with their 25-year-old coach.
The boys left their backpacks and shoes "before wading in and trying to go to the end of the tunnel, sort of like an initiation for local young boys to… write your name on the wall and make it back" said Ben Reymenants.
"Now a flash flood because of sudden heavy rain locked them in," he added.
Mr Reymenants said that the boys were stable and mentally fit, but weak due to a lack of food.
They had been surviving by drinking water off the walls of the cave. The children and coach had no access to food before rescuers reached them.
The group had one flashlight which had ran out.
Two navy SEAL doctors have been with them and helping to get their strength back up.
Journeying into the cave "was very taxing, especially with the emotional load of the lives of 12 young boys," Mr Reymenants said.
The dive was "one of the more extreme cave dives I've done", he said, crediting the complexity, the current, the poor visibility and distance for the difficulty of the expedition.
The leading option to rescue the boys, who do not know how to swim, is to teach them to scuba dive and lead them out of the caves.
Asked if it would be viable to set the children and coach up with breathing equipment and to strap them to experienced divers, Mr Reymenants pointed out that aside from the dangers if one of the boys panicked, some segments of the cave are only wide enough for one person to fit through.
Mr Reymenants and his team were taking turns with a another team to lay down guide lines to help the divers find their way back out and to ease future trips into the cave.
The teams used a 30-year-old map to navigate their way through the complex cave systems.
Meanwhile the country's interior minister has warned the evacuation of the trapped boys "must speed up".
Anupong Paojinda says a rescue attempt is to be made as soon as possible before more rain falls and exacerbates the flooding in the cave.
"As rain is forecast in the next few days, the evacuation must speed up. Diving gear will be used. If the water rises, the task will be difficult. We must bring the kids out before then," he is quoted as saying in The Bangkok Post.
Mr Paojinda acknowledged the difficulty of getting the boys and their coach to dive out of the cave.
He said: "Diving is not easy. Those who have never done it will find it difficult, because there are narrow passages in the cave. They must be able to use diving gear. If the gear is lost at any moment, it can be dangerous to life."
The paper also reported that Mr Paojinda said the boys did not know how to swim, which further complicates efforts to extract them from the cave.
Rescuers have appealed for a donation of 15 small-sized full face masks for the boys in the cave.
The comments come as it is announced that police will look into whether the 25-year-old coach of the football team could face legal action for leading them into the cave, according to Khaosoenglish.com.
The 12 boys and their coach were trapped in the cave for 10 days before rescuers reached them.
The team are in a good condition but in need of nutrition. Their eyes, not used to light after being in the dark cave for days, would need to be protected as the trapped people emerge.
The divers that reached the trapped group were British and captured the moment on video.
The rescuers included British divers John Volanthen and Rick Stanton, and a team of Thai navy SEAL divers.