The 12 missing boys and their football coach have been found in Tham Luang by a British cave explorer - all alive and apparently safe.

A dramatic video from the Pattaya Beach area deep inside the cave indicated an English cave explorer was the first to reach the boys.

The group, mostly seated and with football shirts pulled over their knees, asked for food and to leave the cave immediately, according to the video, which was shared on the official Facebook page of the Royal Thai Navy Seals.

It showed all 13 members of the team moving, alert and with questions: "How long have we been here?" and "When will we get out?"

"Many, many people are coming," said the English rescuer. "We are the first. Don't worry now."

The dramatic footage above was shared on the official Facebook page of the Seals, the nickname of the elite Royal Thai Navy Underwater Demolition Team (UDT).

It shows the boys in muddy football shirts crowded together on a small mud cliff surrounded by water as rescuers finally found them.

"Thank you!" one of them says, while another shouts "13" when asked by a British diver how many they are.

One of the British divers who found the group replies simply: "Brilliant!"

"Many, many people are coming... we are the first", a diver tells them as he lights the group with a torch.

"You have been here for 10 days, you are very strong," one of the divers tells the boys.

A child in the foreground appears to bow in gratitude and says "thank you" as his voice falters.

Chiang Rai governor Narongsak Osotthanakorn said they were found in the area near the so-called Pattaya Beach and rescuers were planning late Monday night to get them out of the cave.

But first, "We will bring food to them and a doctor who can dive. I am not sure they can eat as they have not eaten for a while."

Divers reached the group after enlarging a narrow, submerged passageway that was too small for them to get through while wearing their air tanks.

Earlier, a team of rescuers had used huge pumps to reduce the water level, and divers had placed guide ropes and air tanks along the route to reach the site of the trapped boys.

Another round of hard work now begins to get them out of the flooded and cramped cave.

"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," Narongsak said.

Anmar Mirza, a leading American cave rescue expert, said many challenges remain for the rescuers. He said the biggest decision is whether to try to evacuate the boys and their coach or to supply them in place.

"Supplying them on site may face challenges depending on how difficult the dives are," Mirza, coordinator of the US National Cave Rescue Commission, said in an email.

"Thai Navy seals have found all 13 with signs of life," the governor told reporters. "We found all 13 safe... we will take care of them until they can move."

"Our first mission is accomplished. Now we are trying to get them out," the governor added.

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.

Despite the careful wording by Mr Narongsak, all early indications were that the boys and coach have survived their ordeal without major physical problems.

The team have been trapped in the cave in Mae Sai district, Chiang Rai, since June 23.