The health of the five people still trapped inside a flooded Thai cave is "still good", the head of the rescue mission has said, after confirming that four more boys were brought out safely from the cave complex today.

A 12-member football squad and their coach were trapped inside the Tham Luang caves for more than two weeks.

Eight have been rescued so far in an operation that began yesterday.

"We have helped four more children today," said rescue chief Narongsak Osottanakorn.

He said rescuers would resume an operation to retrieve the remaining five people in about 20 hours' time.

"The health of the remaining five people inside the cave is still good," he said.

Officials said all four boys rescued in today's operation have arrived at hospital and all are safe, and that today's operation went smoothly and that the next phase "will depend on all conditions".

Looming rain was one of the main enemies of the operation, threatening to flood the cave complex in mountainous northern Thailand.

Thais have been fixated on the crisis, hoping desperately for the safe return of the 12 boys and their 25-year-old coach, since they became trapped in the Tham Luang cave complex on 23 June because of rising waters.

They spent nine days unaccounted for inside the cave, before British divers found the emaciated and dishevelled group huddling on a muddy bank above the flooding.

Yesterday, four members of the team were successfully brought out from the cave, after authorities decided they had to rush ahead with a rescue operation to beat monsoon rains.

They were guided by expert divers who plotted the hours-long escape through more than 4km of twisting passageways and flooded chambers.

Officials said all four boys are fine but are being kept apart from relatives for the moment because of infection concerns.

"We've been working continuously overnight," a Chiang Rai government source told AFP, requesting anonymity, and confirming that there had only been a pause of the actual extraction operations.

Authorities continued to refuse to release details about the identities or conditions of the four who had escaped.

"Don't ask these kinds of questions. Not wise questions to ask," Prime Minister Prayut Chan-O-Cha told reporters in Bangkok when asked about the conditions of the four.

With so few details released, parents continued their agonising wait to be reunited with their sons.

"I am still waiting here at the cave, keeping my fingers crossed to see whether my son will be one of those to come out today," Supaluk Sompiengjai, a mother of Pheeraphat - known by his nickname "Night" said.

"We heard four boys are out but we do not know who they are. Many parents are still here waiting. None of us has been informed of anything."

But she added she was "happy" at the prospect of seeing her son again.

To get the remaining boys out, divers will be forced by the narrow passages to accompany them one at a time.

None of the boys have scuba diving experience and experts have warned they could easily panic while swimming underwater in darkness.

The lack of space has added complexity to storing enough canisters of oxygen along the route out.

The death of a former Thai Navy SEAL diver who ran out of oxygen in the cave on Friday underscored the danger of the journey even for professionals.

Weather forecasters said heavy rain could hit the area this afternoon and continue through the week. 

Authorities have repeatedly warned that the rain could re-flood crucial parts of the cave complex that have been drained and make the escape route march harder or even impossible to navigate.

The hordes of global and local media have been kept back from the cave and the hospital in Chiang Rai, where the rescued boys are believed to be under observation.

Night's relatives have said they believe the group went to the cave to celebrate his 16th birthday after a Saturday football practice and got caught as heavy rains caused the water inside the cave to suddenly rise.

A frantic rescue mission was hatched in the week since they were found.

Expert climbers, divers and Thai Navy SEALs have mulled contingencies ranging from drilling an escape route through the mountain to waiting out the monsoon inside the cave. 

But the rescue was prodded into action by the threat of a fresh round of rains and falling oxygen levels in the cave.