Twelve boys and their soccer coach trapped in a waterlogged cave complex in northern Thailand have been learning diving techniques but are not yet ready to swim to safety, Chiang Rai Gov. Narongsak Osottanakorn said early Saturday.

Meanwhile, handwritten messages from the 12 schoolboys and their 25-year-old soccer coach to their families have been brought out of the cave by Thai navy divers.

(Divers head toward the cave complex on July 3)

Rescuers have agreed that diving is the best way to get them out from the Tham Luang Nang Non complex, Narongsak said. "They are learning how to dive and how to breathe" but "cannot dive at this time."

While drilling from the mountainside continues to gain access to the cave where the boys and their coach are trapped, the main plan remains to extract them through the cave entrance when it can be done with minimum risk, he said.

Only 18 of around 100 drilled holes have the capacity to go deeper and the angles do not match the location of the soccer team.

The main concern is rain forecast in the next three to four days. If heavy rain causes massive flooding inside, an early extraction will take place, according to Narongsak.

The governor spoke following the death of a former navy SEAL during the rescue effort. Samarn Kunan, 38, lost consciousness due to a lack of oxygen while diving.

Thai and foreign rescuers have been grappling with how to extract the group safely from deep inside the flooded cave complex while they battle against time.

Related coverage: 

Thai diver dies while working in cave rescue

Rescuers grappling with how to extract 13 trapped in Thai cave

Boys found in Thai cave say they are in good health

The handwritten messages from the boys and the coach to their families were on Saturday posted on the Thai Navy SEAL Facebook page.

One of the boys told his parents "Don't worry about me. I've been away for two weeks, but I'll return to help at the shop (family business)."

Another boy whose nickname is Nick said he looks forward to having his favorite food after getting out of the cave.

"Even though it is quite cold here, don't worry I'm fine. Don't forget to hold a birthday party for me," the boy, nicknamed Dom, told his parents in the message.

The soccer coach apologized to the boys' families for the incident and promised to take care of them as best as he can.

The 12 boys, aged between 11 and 16, and their 25-year-old male coach went missing after entering the cave on June 23 after soccer practice.

They were found alive deep inside the cave on Monday and have since received food and medical treatment.