The boys have now been underground for two weeks. - Photo: Facebook/Thai NavySEAL
The Thai boys trapped in a cave for two weeks have written letters to their parents, saying "don't worry... we are all strong".
The boys' handwritten notes include requests for different types of food.
"Teacher, don't give us lots of homework!" read one. The team's coach also offered his "apologies" to the parents in a separate letter.
In his letter, the 25-year-old coach Ekkapol Chantawong reassured parents, saying: "Dear all kids' parents, now all of them are fine, the rescue team is treating us well.
"I promise to take care of them in the best way and thanks for all the kind words. I also sincerely apologise to the kids' parents," he added.
The notes are the group's first communication with their families, after attempts earlier this week to establish a phone line inside the cave failed.
Thai rescuers are racing against the clock to rescue 12 boys and their football coach in the Tham Luang cave complex, before Sunday's forecast of heavy monsoon rains hit.
The expected deluge could force the water level up, threatening to flood the pocket where the group is currently residing.
A former Thai navy diver has already died due to a lack of oxygen, while delivering supplies to the group.
Massey University's Associate Professor Sarb Johal, from the Joint Centre for Disaster Research, said the lack of daylight could leave the group psychologically disoriented.
"Many of their basic physiological functions that depend upon circadian rhythms will be disturbed, such as sleep, hormonal functioning, core temperature, their feeding and drinking, and also how their gastrointestinal systems might be functioning," he said.
If the boys had to stay there for an extended period of time, he said circadian-based lighting in the cave could help and establish a sense of hope.
"That may help to regularise some of these bodily functions and go a long way to develop some kind of routine in day-to-day living in this extraordinary situation."
Problems with sanitation, through a compromised immune system due to malnutrition and a lack of sleep, would not help the situation.
He said safe removal of faeces would be important, and there might be other sources of infection in the underground cave system.
But other than mitigating the physical hazards, support for one another in the cave would also be crucial, Mr Johal said.
However, situations might become fraught if they had to stay together for an extended period of time.
"It's hard to know how it will play out but critical points may emerge, such as who may be selected to attempt escape first and how those choices are made. There are no easy answers to such dilemmas, and there may be many more to come," he said.
Rescuers are trying to establish a line of communication with loved ones for those trapped.
And ultimately communication with the outside world and the group's loved ones was one of the best ways to encourage their survival, said Mr Johal.
Meanwhile, an air line has been installed to the cave where the group has been trapped.
Speaking to reporters on Friday, the governor of the Chiang Rai region, where the cave is situated, said the boys had enough strength to walk but could not swim to safety.
Narongsak Osotthanakorn said the health of most of the boys had "improved to normal", and that divers were continuing to teach diving and breathing techniques.
When asked if a rescue attempt would be made overnight if it started to rain, he said: "No, the boys can't dive at this time."
Mr Narongsak said the boys were "OK" and the air they were breathing was good.
On Thursday their parents sent letters in for the boys to read - but the governor said he was not sure if these had reached them yet.
Rescuers have dug more than 100 holes, hoping to reach the cave by a direct route. There were 18 promising ones, the deepest at 400m, but Mr Narongsak said he was not sure if it would reach the boys, who are believed to be about 600m below the surface.
What is the situation?
The group was found in the cave by British rescue divers 10 days after they went missing. They were perched on a rock shelf in a small chamber about 4km (2.5 miles) from the cave mouth.
Teams of Thai and international divers have since supplied them with food, oxygen and medical attention, but there are mounting concerns about the oxygen level in the chamber, which officials said had fallen to 15 percent. The usual level is 21 percent.
Mr Narongsak said discussions regarding the lowest risk rescue attempts were ongoing, but acknowledged that time was against them.
"We will try to set the best plan, when the risk is minimal, we will try to bring them out," he said.
He added that the rains had stopped in the past few days, giving the rescue operation some space to make several areas in the cave dry enough for rescue workers to access.
He also said that when teams had stopped pumping water for 12 minutes during the day on Friday, the water level in the cave rose by 10cm.
On the surface, a huge military and civilian rescue operation is racing against the clock to bring the boys to safety. Heavy monsoon rains are expected on Sunday, threatening further flooding.
Officials had initially considered leaving the boys in the chamber to wait out the rainy season - which could have seen them trapped there for up to four months.
- RNZ / BBC