A haunting reminder of how young they are: The letters boys are sending parents from Thai cave.

With AAP. 

The soccer coach trapped in a cave with 12 Thai boys has apologised to their parents in the first letter he and the team have sent out through divers, in which the boys say they are doing well and missing their families.

Rescuers say they will not immediately attempt an underwater evacuation because the boys have not yet learned adequate diving skills.

However, if heavy rains start again, divers will try to take the boys out right away, Chiang Rai Governor Narongsak Osatanakorn says.

The same day saw a disheartening setback with the death of a former Thai navy SEAL diving in flooded passageways to deliver oxygen supplies.

“To the parents of all the kids, right now the kids are all fine, the crew are taking good care. I promise I will care for the kids as best as possible. I want to say thanks for all the support and I want to apologise to the parents,” wrote 25-year-old Ekapol Chanthawong, the coach of the Wild Boars soccer team.

One boy writes: “I’m doing fine but the air is a little cold but don’t worry. Although, don’t forget to set up my birthday party.”

Sydney Morning Herald reports that other letters said, “they want to go straight to their home,” and that they asked their teachers for not too much homework.

The boys, aged 11 to 16, and their coach went exploring in the cave after a soccer game June 23.

Monsoon flooding cut off their escape and prevented rescuers finding them for almost 10 days.

The only way to reach them was by navigating dark and tight passageways filled with muddy water and strong currents and in oxygen-depleted air.

Asked at news conference early on Saturday about bringing the boys out underwater, the governor replied: “Not today because they cannot dive at this time.”

Narongsak said the boys were still healthy and had practised wearing diving masks and breathing in preparation for the diving possibility.

Thai officials had been suggesting a quick underwater evacuation because of the possibility that access to the cave could soon close again due to monsoon rains expected this weekend.

Cave rescue specialists have cautioned against that approach except as a last resort because of the dangers posed by inexperienced people using diving gear.

The suggestion the trapped team might have to wait months inside until a safe way out is available, as was the case in 2010 with Chilean miners trapped underground, has met little enthusiasm.

Authorities continue to pursue a third option, which is finding a shaft or drilling into the mountain to find a sort of back-door entrance.

The death of Thai diver Saman Gunan underscored the risks of making the underwater journey.

The first fatality of the rescue effort, he died while placing oxygen canisters along the route to where the boys and others are sheltered.

A dozen Australian defence and police specialists have joined in the rescue.