A major operation is under way to reduce the flow of water into the cave where 12 Thai footballers are trapped with their coach.
A weir has been constructed further upstream from the mountain to divert the water currently rushing into the Tham Luang Cave system.
Experts say plans to teach the youngsters how to scuba dive so they can swim out underwater are fraught with difficulty.
The submerged stretch is 1.2 miles (1.9km) long and contains a very narrow passage that is only wide enough for one person.
The boys, aged 11-16, and their 25-year-old coach were found deep inside the cave on Monday, nine days after they went missing following a football game on 23 June.
Sky News’ Lisa Holland, who is at the scene near Chiang Rai, in the country’s far north, said getting the boys to dive out of the caves was not the rescuers’ number one option.
Thai authorities are looking at several possibilities depending on the water levels inside the caves and the weather, she added.
Much work has gone into stemming the flow of water into the cave and pumping out the water that is already there, in the hope the boys may not need to dive and if they do, it will be for a shorter distance.
Chongklai Voraphongston, deputy director-general of the Department of National Parks, Wildlife and Plant Conservation, said Huai Nam Dan stream, which runs into the caves, has been completely blocked.
The Huai Makok stream meanwhile, which is located to the south of cave and flows into it, has been diverted, using six 200-meter-long (656ft) pipes.
Several natural shafts, which allow water to pour into the cave system from above, are being plugged.
Major General Bancha Duriyaphan said the volume of water draining from the cave had fallen and was expected to decrease further provided there is no rainfall.
The governor of Chiang Rai province said the extraction of the football team and their coach would depend on their physical condition.
Narongsak Osatanakorn said that “all 13 may not come out at the same time”.
He added: “If the condition is right and if that person is ready 100%, he can come out.”
Mr Osatanakorn said that a new cave, suspected to be connected to the area where the group are located, has been discovered and was being explored.
Video footage released overnight showed the group appearing to be in reasonably good health, with a team of Thai navy SEALs treating them for minor injuries on their feet as the boys smiled and made gestures to say “hello”.
One of the boys and the 25-year-old coach are said to be suffering from “medium” health problems, but they have all been provided with reflective blankets to keep them warm and further supplies of food.
A team of British cave divers, two of whom were the first to locate the footballers, are continuing to help move food and equipment to the place where the team are waiting, which is known locally as Pattaya Beach.
The boys are being looked after by seven navy SEALs, including medics, who are staying with them inside the cave.
Two of the boys are wearing English football shirts – one the red away kit of the national side and the other a Chelsea top.
SEAL commander Rear Admiral Arpakorn Yookongkaew said there was no rush to bring out the group, as they are safe where they are.
A fibre optic cable is being installed along the underwater passage to the boys, which will allow rescuers to talk to the group and anyone with them.
But heavy rain is forecast for this weekend which could force authorities to decide the boys should swim and dive out – something rescue experts have said could be extremely dangerous.