Four Thai boys have been rescued so far from a flooded cave where they were trapped with their teammates and soccer coach.
The Thai navy told the Associated Press four of the stranded boys have been brought to safety.
Two ambulances were earlier seen leaving the cave and a helicopter was seen taking off, AP reported.
Officials previously said helicopters were on standby to take anyone rescued to hospital.
Chief of operations governor Narongsak Osottanakorn said 13 foreign and five Thai divers are taking part in the operation.
The divers entered the Chiang Rai cave on an 11-hour round trip to rescue the 12 boys and their soccer coach at 10am on Sunday (1300 AEST).
They are planning for two divers to accompany each boy back to the surface through the four kilometres of tight, muddy, water-filled passageways, some of which are less than a metre in diameter.
Two divers will accompany each boy as they are gradually extracted.
Six of the divers are with the Australian Federal Police, while another is an anaesthetist from Adelaide along with his diving partner.
A spokesperson for Foreign Minister Julie Bishop said the anaesthetist entered the cave on Saturday to check the boys' health and has been working closely with on-site doctors.
Australian media at the scene said the anaesthetist was called in at the request of British divers on the scene who were familiar with his expertise.
He has extensive cave-diving experience and it's understood he's acting as a private citizen.
The entire operation to get all 13 out of the cave could take days, depending on weather and water conditions, Thai army Major General Chalongchai Chaiyakam said.
A doctor and paramedics are standing by to provide medical attention whenever the rescue team emerges.
Rescuers pumped millions of litres of water out of the cave network to try to extract the group through nearly a mile of tunnels.
Thai officials are not only racing against worsening weather, but also lowered oxygen levels in the underground complex in the north of the country.
Former Thai navy Seal Saman Kunan passed out making the dive on Friday and died.
The boys, aged 11-16, and their coach, 25, were trapped inside the Tham Luang Nang Non cave in the northern province of Chiang Rai after a football game on June 23.
Monsoon flooding cut off their escape and they have been on a rocky shelf inside a cave chamber since.
Although weak, they are largely in good health, authorities have said.