Image copyright Facebook/ekatol Image caption A Facebook photo shows the coach with some of the missing children

Twelve Thai boys trapped in a cave have exchanged emotional letters with their parents, for the first time since their ordeal began two weeks ago.

"Don't worry, we are all strong," one child said in a note. "Teacher, don't give us lots of homework!" he joked.

The football coach who took the boys into the cave apologised to parents, but they wrote he was not to blame.

Officials say that they have a window of up to four days before rains will make the rescue more difficult.

The boys were exploring the cave when they became trapped by floodwaters on 23 June.

What do the letters say?

The handwritten letters from the group were handed to British divers on Friday and released on the Thai Navy Seal Facebook page on Saturday.

"Don't worry about me, I'm safe." a boy nicknamed Pong said.

Another wrote: "Nick loves Mum and Dad and siblings. If I can get out, Mum and Dad please bring me mookatha (Thai barbecue) to eat."

In his letter, the 25-year-old coach Ekkapol Chantawong, also known as Ake, said: "Dear all kids' parents, now all of them are fine, the rescue team is treating us well.

"And I promise I will take care of the kids as best as I can... I also sincerely apologise to the kids' parents."

In their letters, posted on the local government's Facebook page, several parents make clear that they do not blame the coach.

"Mums and dads are not angry at you. Thank you for helping take care of the kids," one said.

Another reads: "Tell Coach Ake: Don't think too much. We are not angry at him."

It was the first communication since attempts to establish a phone line inside the cave failed earlier this week.

'He writes just like his mother'

Helier Cheung, BBC News, Maesai district

The letters make for remarkable reading. Words of love, reassurance and encouragement are interspersed with talk of food the boys are craving - and birthday parties.

At least two of the boys have had their birthdays while stuck inside the cave.

One of them, Note, has just turned 15.

At the MyWay garage, where Note's father works, some of the workers have known Note since he was born - and describe him as a smart kid who loves sport and helps his father with mechanical work over the holidays.

They were touched by his letter - but still full of concern for the boys.

One of them, Rinlinee Sombat, told the BBC: "He writes just like his Mum. But I want to see him even more than I want to see his handwriting."

Image caption Rinlinee Sombat and Ponrawee Tachavandee are hoping for Note's safe return

Meanwhile, Ponrawee Tachavandee said he had spoken to Note's mother on Friday night - and she had been distraught since learning about the death of a Thai navy diver that day.

He said she had told him that the "navy Seal had practised for so long, and was so strong, but also died. How about a boy who has never dived before?"

What is the situation inside the cave?

The boys were found inside the cave by British rescue divers on Monday, 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.

There are concerns about the falling oxygen level in the chamber, but officials say an air line into the cave has now been installed.

Media playback is unsupported on your device Media caption Dr Alex Rowe explains the impact of being trapped underground

The danger of their situation became clear when a former Thai navy diver died after delivering air tanks to the group on Friday.

When could the group be rescued?

Above ground, a huge military and civilian operation is racing against the clock to bring the boys out. Monsoon rains are threatening further flooding in the coming weeks and month.

On Saturday, the governor of the Chiang Rai region, where the cave is located, said the next three to four days were "the most favourable time for the operation in terms of the water, the weather and the boys' health".

Narongsak Osottanakorn said that after that, further rains could imperil the group.

"The water level may rise to the area where the children are sitting and make the area less than 10 sq m [100 sq ft]," he said.

Another concern, he added, was the growing concentration of carbon dioxide exhaled by the boys and rescue workers.

Media playback is unsupported on your device Media caption Heavy rain is not expected until late in the week in the area where the boys are trapped in a cave

He gave no details about how the rescue might be carried out.

Meanwhile, rescuers outside have dug more than 100 holes in an attempt to reach the cave by a direct route.