The same divers who took part in Sunday's rescue of four boys trapped in a flooded cave in Thailand will be returning to bring out the remaining nine.

The experienced divers have been entrusted with the complex and risky operation as they know the cave conditions and what to do, Thailand's Interior Minister Anupong Paojinda said.

Mongkol Boonpiem, 13, was the first boy to be rescued. Picture: Thaivisa Français (Supplied)

It comes as sources confirm to The Guardian that the divers have not yet gone into the cave but will go in "soon", with at least seven ambulances en route to the site.

Thai officials met this morning about the next stage of the operation and how to extract the remaining eight boys and their soccer coach from the cave in the country's north.

The weakest boys were plucked from the depths of the Thai cave first, with an Australian doctor likely behind the push.

Parents of the trapped children wait patiently for their boys to be rescued from the cave. Picture: Getty (Getty)

Rescue crews load one of the boys into the back of a van before he is taken to Chiang Rai Hospital. Picture: AAP (AAP)

While the initial plan was to start with the strongest boys, cave diver and anaesthetist Richard Harris, 53, from Adelaide assessed the youngsters and their coach and that strategy was reversed, Thai media has reported.

Four boys have so far been successfully extricated from the cave, with rescuers taking advantage of low water levels.

Four boys are in hospital after being freed from a flooded Thai cave. Picture: AAP (AP)

Heavy rains are approaching the Thailand caves where eight boys remain trapped. Picture: Supplied (AP)

It is understood the boys were able to wade through some sections of the underground labyrinth instead of diving.

Oxygen tanks are being refuelled as divers prepare to go in again.

Chiang Rai acting governor Narongsak Osottanakorn said the rescue operation, launched early yesterday afternoon, had been completed "hours faster than expected".

The rain is expected to cause problems for rescuers as they move into day two. Picture: Supplied (AP)

The first boy taken from the cave was Mongkol Boonpiem, 13, who emerged at 5.37pm local time (8.37pm) and was taken to Chiang Rai Prachanukroh Hospital.

Thai media reported one of the four boys was in a critical condition and a family friend told a reporter it was Mongkol, the weakest of the group, whose condition has since stabilised.

"I think Narongsak and the commanders changed their minds after the Australian doctor inspected the boys' health and mental state," one senior Thai journalist said.

One of the boys trapped in the cave, in a photo taken last week. Picture: Supplied (Nine)

"They decided to take the weakest out first."

The boys, aged 11 to 16, are members of the Mu Pa Academy Mae Sai football club and have been trapped inside the Tham Luang cave with their coach, Ekapol Chantawong, 25, since June 23.

Dr Harris is among a number of Australians assisting the rescue operation.

He has 30 years' diving experience and was involved in the retrieval of the body of stunt diver Agnes Milowka who died at Millicent, South Australia, in 2011.

Eight Australian divers, including six from the Australian Federal Police, are part of the 90-strong Thai and international diving team taking part in the operation.

Foreign Minister Julie Bishop has welcomed the rescue of four boys from the cave, as 19 Australians continue to work on the precarious rescue operation.

Ms Bishop said the Australian contingent had been integral in the first stage of the rescue mission, with the eight remaining boys and their soccer coach still in the cave waiting to be extracted.



"It's highly dangerous, it's very precarious and our thoughts are not only with the boys but also with the diving and rescue teams that are assisting," Ms Bishop told the TODAY Show.

It's understood the decision to ban the media, totalling about 1000, from the area around the cave entrance yesterday was to facilitate faster ambulance access to the freeway to get the rescued boys to a football field where they could be taken by helicopter to the Chiang Rai hospital.

Prime Minister Prayut Chan-o-cha is scheduled to visit Chiang Rai this afternoon.