The coach of a youth football team trapped for two weeks in a flooded Thai cave has sent his "apologies" to their parents in a scrawled note handed to divers, as officials Saturday appeared to rule out an immediate rescue.

With fresh monsoon rains forecast to arrive within a day and fears mounting over the reduced oxygen and high level of water in the cave, Thailand is holding its breath for the safe return of the group.

Ekkapol Chantawong, 25, was for nine days the only adult with the children -- aged 11 to 16 -- until they were discovered on a muddy ledge by rescue divers on Monday.

"To all the parents, all the kids are still fine. I promise to take the very best care of the kids," he said in a note given to a diver on Friday and published on the Thai Navy SEAL Facebook page on Saturday.

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"Thank you for all the moral support and I apologise to the parents."

Other touching notes signed by members of the "Wild Boar" team were directed to their families, many of whom have kept an anxious vigil outside the cave for a fortnight.

Handwritten messages written by boys and their soccer coach who are trapped in the Tham Luang cave in Chiang Rai, are seen in this combination of undated photos obtained from social media. Photo: Reuters

"Don't worry dad and mum," said a note from one of the boys who gave his nickname as "Bew".

"I have been away for two weeks but will come back and help you sell your stuff." His family are shopkeepers.

Another note, from "Dom" said he was "fine but the weather here is a little cold".

The boys are being trained in the basics of diving in case the floodwaters force authorities into a sudden evacuation through twisting and jagged passageways.

In an update in the early hours of Saturday morning, rescue operation chief Narongsak Osottanakorn said it was "not suitable" to make the boys dive to safety yet.

However, he indicated that further downpours might speed up attempts to extract them, despite the dangers.

'Limited time'

The risks were underlined by the death on Friday of a former Thai Navy SEAL diver, who ran out of oxygen while returning from the chamber where the boys are trapped.

Saman Kunan had been trying to establish an air line to the chamber when he passed out and perished.

"We lost one man, but we still have faith to carry out our work," Navy Seal commander Apakorn Yookongkaew said, adding that rescuers have "a limited time" to extract the boys.

Ekkapol's message is the first from the coach, whose role in the team's predicament has split Thai social media.

Many have lauding him after reports he gave his share of food to the kids before they were located and helped them get through nine days in the darkness.

Others have criticised him for agreeing to take the young boys into the cave during the monsoon season.

The group entered the cave on June 23 and got trapped as floodwaters tore in.

A massive rescue operation is under way, with high-powered pumps working round-the-clock to reduce the water level inside the cave, while rescue experts weigh their options to extract the kids.