A Thai rescuer has died after falling unconscious during part of an operation to rescue 12 boys and their soccer coach trapped inside a cave complex in northern Thailand.

Samarn Poonan, a former member of Thailand's elite navy SEAL unit who was part of the rescue team in Chiang Rai, died on Thursday night after entering the cave to lay oxygen tanks along a potential exit route, the SEAL commander said.

Rescuers in Thailand were no closer on Thursday to deciding when and how to extract 12 boys and their soccer coach, hungry and frail but otherwise in good health, after nine days lost underground.

The search and rescue operation for the junior soccer team, who disappeared in the cave in the northern province of Chiang Rai on June 23, had seemed to be nearing an end when British and Thai divers found the boys late on Monday, huddled on a small, muddy bank in a flooded chamber.

But rescuers now have to figure out how to get them out, through several kilometres of dangerously flooded tunnels, or even to attempt to bring them out for now.

One possibility is that the 13 stay put in the Tham Luang cave until the flood waters recede, at the end of the rainy season in about four months.

But some officials say the boys could be out in days if the weather is on their side and enough water can be pumped out of the cave network to enable the boys to get out the same way they got in, just before heavy downpours hit the region, on foot through muddy tunnels, perhaps with some swimming.

A third option would be to teach the boys to use scuba gear, and they then dive through the flooded tunnels, the way their rescuers reached them. A fourth possibility would be to find an alternative way into their chamber.

Kobchai Boonarana, deputy director-general of the Disaster Prevention and Mitigation department, said it was up to the rescue team in the cave to decide if and when the boys would be strong enough to tackle the journey out.

"Their conditions, we can see that their morale is good but what about their strength and their ability? That's up to the team inside to decide," Kobchai told reporters on Thursday.

"Our job is to keep pumping out water and it is up to the team inside to assess the safety level and whether the kids can travel safely through," he said.

Regional army commander Major General Chalongchai Chaiyakum, said it took rescuers 11 hours to do a round-trip from the cave's entrance to the group and back, often battling powerful currents in the murky water.

Rescuers had to contend with days of heavy rain that flooded the cave complex at the beginning of the search but the weather has been relatively dry for the past four days.

But the meteorological department warned that up to 60 percent of the country's north, including Chiang Rai, can expect heavy rain from July 7 to July 12.

Authorities have sent in food, water and medical staff, some of whom stay with the boys full time.