A foreign diver involved in the mission to save 12 boys and their football coach from a flooded Thai cave has hailed the children as "incredibly strong", and described their treacherous escape journey as unprecedented.

"They are getting forced to do something that no kid has ever done before. It is not in any way normal for kids to go cave diving at age 11," Ivan Karadzic, who runs a diving business in Thailand, told the BBC.

"They are diving in something considered (an) extremely hazardous environment in zero visibility, the only light that is in there is the torch light we bring."

The boys, aged from 11 to 16, and their 25-year-old coach, ventured into the Tham Luang cave in northern Thailand on 23 June after football practice and became trapped when heavy rains flooded the cave.

Two British divers found them nine days later huddled on a muddy ledge in pitch darkness more than 4km inside the cave system.

Authorities then gathered 90 divers, 50 of them foreigners, to help extract the boys out of a claustrophobic tunnel network that in some places was completely filled with water and so narrow that they could only be squeezed through.

Conditions were so dangerous that a retired Thai Navy SEAL died on Friday while trying to lay out oxygen tanks underwater in a tunnel, and the rescue chief at one point dubbed the operation "Mission Impossible".

Adding to the dangers, most of the boys could not swim, and none had scuba diving experience.

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Timeline: Thai cave rescue

Mr Karadzic, who the BBC reported was stationed near a difficult stretch of the cave about halfway along the escape route to replace oxygen tanks and help guide people through, said the rescue workers had feared the worst.

"We were obviously very afraid of any kind of panic from the divers," he said, adding he was in awe of the boys' ability to stay calm.

"I cannot understand how cool these small kids are, you know? Thinking about how they've been kept in a small cave for two weeks, they haven't seen their mums. Incredibly strong kids. Unbelievable almost."