As four members of the Thai soccer team trapped inside a cave are freed, the huge operation to rescue the remaining boys is still under way.

The Tham Luang cave complex poses a range of problems for rescuers as they aim to extract the eight boys and 25-year-old coach left in the mostly-flooded, snaking system in the coming days.

The trip out with the boys takes almost eight hours, after it took professional divers about three hours to reach the group, and another five to get the boys out.

It was previously said to take rescuers 11 hours to complete a round trip – six hours there and five back.

The boys, aged 11-16, and divers have to wade and dive through floodwaters, and climb through caverns and crevices, with one point just 38cm wide.

The boys must traverse one dive section before they are handed to a second group of rescuers to help them climb about two kilometres to the cave entrance.

The journey the boys must take out of the cave will take five hours. Photo: Twitter

Details of the delicate operation were released by the Thai government as rescue efforts began again at 10am (AEST) on Monday.

Two divers will accompany each boy, who will be guided through the dark cave and murky waters using ropes.

In the narrow sections, divers are forced to take off their air tanks and slip them through.

Each boy will also be given a full face mark – which is thought to make the difficult dive easier for novices – and will be tethered to a diver.

Thai government releases graphic about #thamluangcaverescue . Full face masks; 2 divers accompanying 1 boy; guided by rope. When facing a very narrow path, they will release the tank from back and slowly roll tank & guide the boy through. They walk from Chamber 3 to mouth of cave pic.twitter.com/pLUKa8lHfd

— Nick Beake (@Beaking_News) July 8, 2018

Concerns for the boys were emphasised after the death of a Thai navy diver, who was part of a team trying to supply the boys’ cave with air tanks.

Officials were also concerned with the oxygen level in the space where the boys were trapped, saying at one point that the level of oxygen in the air had fallen to 15 per cent from the usual 21 per cent.

The team has transferred 100 tanks to the cave to improve air supply.

Chiang Rai provincial governor Narongsak Osottanakorn said the mission was “a race against the water”, as pumping continues to try to reduce the flooding.

Water levels inside were at their lowest levels when the four boys were rescued, but water is being fed by sinkholes and streams in the hills above, and the risks from the imminent monsoon rains are high.

“Our biggest concern is the weather. We are calculating how much time we have if it rains, how many hours and days,” Mr Narongsak told reporters.

Weather reports indicate that heavy rain is expected on Monday night, potentially raising the level of water in the cave as run-off flows makes its way into the underground chambers.

-with ABC