Former Chiang Rai province governor Narongsak Osottanakorn, who is still in charge of the rescue effort, has confirmed four of the boys have been rescued. After their health was evaluated, they were sent to Chiang Rai hospital together.

The governor said he could not say when the next rescue operation would take place, but expected it would be within 10 to 20 hours, “not over 20 hours”, after evaluating all the factors and ensuring that everything is ready, including all ambulances, helicopters and medical teams as well as the equipment of the rescue teams.

The first two boys were rescued at 5.40pm and about 5.50pm local time, and the third and fourth boys were rescued at 7.40pm and 7.50pm local time.

The boys reportedly swam one kilometre under water during the operation.

“Ninety divers from around the world helped us today,” he said. “Fifty were professionals from abroad and 40 were Thai locals.

“For the next question, what’s the next plan? We will get ready and make sure that everything is 100 per cent because all of the oxygen tanks and equipment have to be re-filled."

Of the four rescued boys, he said, their health and factors such as the amount of oxygen in their blood would need to be checked.

The Thai Navy Seals confirmed the rescue of the four boys overnight, Australian time, about ten hours after the rescue attempt had gotten underway at 10am, local time, on Sunday.

The first boy rescued from the cave was named as 14-year-old Mongkol "Mark" Boonpiam, according to a local Thai media outlet and The New York Times.

In the notes from the boys to their parents released a couple of days ago, Mark had said: "Are you ok at home mum? Please tell my teacher. Love you mum.”

Ambulances could be heard racing past and rescue helicopters buzzed late into the evening as the huge media contingent from all around the world gathered at the local municipal council offices.

The plight of the 12 boys and the coach who made up the missing "Wild Boars" soccer team, who have been missing for nine days before being miraculously found last Monday night, has riveted Thailand and drawn worldwide attention.

Earlier on Sunday, as he announced the rescue mission had begun, Narongsak said there would be two divers per child during the rescue operation and that they would come out of the cave one by one.

"Today we are most ready, today is D-Day. At 10am today 13 foreign divers went in to extract the children along with five Thai Navy SEALs," he said.

We can say they are all international all-stars involved in this diving operation and we selected five of our best [Navy SEALs] who can work with them.”

"We estimate that the soonest they could come out is 9pm [midnight Australian time]. And they will gradually come out as 'a pair'. One will be accompanied by two. It will take time. It’s not that we start at 10am today and everything is done. We will continue the mission until the last one is out."

In fact, the first boy was reportedly rescued just before 6pm, local time – about three hours ahead of schedule.

An Australian doctor with specialist caving expertise, Richard Harris, has played a key role in the rescue operation, giving final medical approval for the operation to proceed on Saturday after examining the boys deep inside the cave complex.

Harris, an anaesthetist from Adelaide, has extensive cave-diving experience and it’s understood his presence was specifically requested by some of the British divers participating in the rescue mission, as they were familiar with his expertise in what is a relatively small cave-diving community.

As well as the Australian doctor, another Australian, a friend of Harris, and six Australian Federal Police divers are involved in the operation - meaning a total of eight Australians are participating.

Heavy rains intermittently lashed the Mae Sai region on Sunday afternoon, underscoring the threat posed to the operation to pump water out of the caves and confirming the urgency of the operation.

Given the difficult conditions, and the ever-present threat of monsoon rains, authorities had warned the operation could be halted at any time.

Water levels in the cave reached their lowest point on Saturday, with another 30 centimetres pumped out of the caves, but Narongsak said that storms were expected in the next couple of days and that "if we don’t start now, we might lose the chance".

Deputy commander of the Third Army Region, Chalongchai Chaiyakum, backed that assessment, warning “it could be two, three or four days. It depends on factors - variable factors like weather conditions and water [levels in the cave]”.

Narongsak said the the kids were very determined and “they are of high spirit. All 13 kids have been informed about the operation and they are ready to come out”.

"They firmly decided to come out with us – confirmed," he said, adding the families of the 13 boys and the coach had agreed to the attempted pull-out.

Military divers and rescue officials had poured into the site in the early hours of Sunday morning, while ambulances and helicopters were on standby to take the boys to the Chiang Rai Prachanukroh Hospital.

Thai Prime Minister Prayut Chan-ocha will travel to visit the families of the boys and oversee the rescue effort on Monday.