Twelve boys and their assistant soccer coach have been found alive by rescuers inside a Thai cave complex nine days after they went missing, Chiang Rai province's Governor announced.

Key points: Group has been given energy gels to keep them going

Group has been given energy gels to keep them going It's unclear whether any are in need of medical attention

It's unclear whether any are in need of medical attention Boys may need to be taught to use basic diving gear in order to get them out

In a video shot by rescuers as the boys were found, flickering torchlight revealed boys clad in shorts and red and blue shirts sitting or standing on the ground in an underground cave beside an expanse of water.

"How many of you are there — 13? Brilliant," a member of the multinational rescue team, speaking in English, tells the boys.

"You are very strong," one of the rescuers says to them in English.

Sorry, this video has expired The boys asked "What day is it?" when they were found.

One of them asks what day it is, and the rescuer responds, "Monday. Monday."

The boys, aged between 11 and 16, went missing with the 25-year-old coach after soccer practice on June 23 when they set out to explore the Tham Luang cave complex in Chiang Rai, the largest northernmost city in Thailand.

A massive international rescue effort has been underway since, with rescue teams battling through thick mud and high water to try to reach the group.

Rescue teams and families celebrated outside the cave when the news was announced. ( Reuters: Soe Zeya Tun )

"Thai Navy SEALs have found all 13 with signs of life," Governor Narongsak Osottanakorn said in comments broadcast nationwide.

"We found them safe. But the operation isn't over."

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Officers from the Australian Federal Police (AFP) were part of the rescue team.

"We are happy and proud to be one part of such an incredible international effort," the AFP tweeted.

It remains unclear whether any of the group are injured or in need of medical attention, but they have been given energy gels to sustain them while a plan is worked out to bring them to safety.

Rescuers had been working on Monday to clear a constricted passageway for divers deep inside the flooded cave complex in a search operation that has been hampered by heavy rain.

Divers from Thailand's elite navy SEAL unit had been focusing on an elevated mound inside the cave which cavers have named "Pattaya Beach", which could have provided the boys with a refuge when rains flooded the cave.

"The SEALs reported that … they reached Pattaya Beach which was flooded. So they went 400 metres further where we found the 13 … who were safe," Mr Narongsak told the cheering group of reporters.

Sorry, this video has expired One mother said she thought her son "only had a 50 per cent chance of survival".

"When the medics have evaluated the kids to see if their health is in good condition, we will care for them until they have enough strength to move by themselves, and then we will evaluate the situation on bringing them out again later," Mr Narongsak said.

Prime Minister Prayuth Chan-ocha thanked the international experts and rescuers who helped locate the missing for their "tremendous efforts".

"The Royal Thai Government and the Thai people are grateful for this support and cooperation, and we all wish the team a safe and speedy recovery," Mr Prayuth's office said in a statement.

Boys may need to use diving equipment

Thai rescue teams work inside the cave complex. ( AP: Tham Luang Rescue Operation Centre )

The next challenge will be getting the group out of the cave, which some have dubbed a "labyrinth".

Rescuers had discussed waiting until water levels subsided to get the boys out.

Other options included teaching the group to use diving gear to navigate the flooded cave.

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Medical teams were seen preparing first aid kits after news of the group's discovery broke outside the cave's entrance.

Relatives of the boys took shelter from heavy rain on Monday and were seen cheering, smiling and receiving calls after hearing the news.

Aisha Wiboonrungrueng, the mother of one of the boys, 11-year-old Chanin Wiboonrungrueng, smiled and hugged her family.

She said she would cook her son a Thai fried omelette, his favourite food, when he returns home.

Rescuers shook hands and congratulated each other as occasional cheers broke out.

It remains unclear whether any of the group are injured or in need of medical attention.

Mr Narongsak said medical teams have been sent inside the cave and it would take them about four hours to assess the group's health and how fit they are before coming up with a strategy to get them out.

Family members were all smiles after hearing the news they had been hoping for. ( AP: Sakchai Lalit )

AP/Reuters