Rescuers braced for a difficult evacuation for 13 members of a Thai youth football team found alive in a cave nine days after they went missing, as a phone cable was hurriedly fed into the underground chambers in the hope of allowing them to speak to their families for the first time since their ordeal began.

The 12 young boys and their football coach were discovered rake thin and hungry on a mound of mud surrounded by water late on Monday, ending an agonising search that captivated a nation.

A team of Thai Navy SEAL divers — including a medic — have joined them on the bank, while rescuers pour over evacuation plans from the Tham Luang cave complex in northern Thailand before heavy rains return and raise the water level.

Much-needed food and medical supplies — including high-calorie gels and paracetamol — reached the group on Tuesday as rescuers prepared for a prolonged extraction operation.

“A telephone line will be installed tonight... they (the boys) will be able to talk with their families via military phone,” Passakorn Boonyarat, deputy governor of Chiang Rai province, told reporters late on Tuesday.

He refused to speculate on how long they might be trapped, but explained that while there are enough provisions for four months, anyone fit and able to leave the cave would be evacuated as soon as possible.

“Any boys who are ready can come out first,” via “chamber three” a cavern being used to as a base to store food, oxygen tanks and diving gear as well as plan the complex logistics of how to move 13 weak and inexperienced divers out of a partially-submerged cave.

Time may be against them given the region's incessant monsoon rains.

'Mission impossible'

The boys were discovered at about 10:00pm (1500 GMT) on Monday by British divers some 400 metres (1,300 feet) from where they were believed to be stranded several kilometres inside the cave.

As the first footage of the kids spread across social media in the early hours of Tuesday, jubilation erupted across a country which has been glued to each twist of the massive rescue operation.

Video posted on the Thai Navy SEAL Facebook page, shows one of the boys asking the rescuers to “go outside”.

In response the British diver says: “No, no not today... many, many people are coming... we are the first.”

“We called this 'mission impossible' because it rained every day... but with our determination and equipment we fought nature,” Chiang Rai governor Narongsak Osottanakorn said on Tuesday.

'Unimaginable' rescue

The harrowing task of getting the boys out is fraught with challenges.

An experienced SEAL diver takes around six hours to navigate the rugged, flooded kilometres-long course towards the entrance.

Experts say the risks of panic, drowning or an accident are high for young, scared and physically drained divers trying to negotiate the tight, winding passages.

If diving proves impossible, there is an outside chance a hole can be drilled into the cave to evacuate them or they will have to wait for waters to recede and walk out — the longest of the options.

Relatives led the outpouring of joy at the dramatic discovery of the boys perched on the muddy bank.

“I'm so relieved, though I still don't have the chance to see him... I want to tell him I'm still here waiting,” Kieng Khamleu, said of her son Pornchai Khamleung inside the cave.

Another parent said he could hardly believe the good news.

“It's unimaginable. I've been waiting for 10 days, I never imagined this day would come,” the father of one of the boys said.

The “Wild Boar” team became trapped on June 23 after heavy rains blocked the cave's main entrance.

Rescuers found their bicycles, football boots and backpacks near the cave's opening, and spotted handprints and footprints further in — leading them to the spot they were eventually found.

Tham Luang cave is one of Thailand's longest, winding 10 kilometres (six miles) and is also one of the toughest to navigate — especially in the wet months.

A sign outside the entrance warns visitors not to enter during the rainy season from July to November.