Chiang Rai governor Narongsak Osottanakorn said all members of the group did not have to be brought out at the same time, but could be extracted over days, depending on the weather.

"We're assessing weather conditions, if it rains and the water level rises, how many hours, days do we have?" he said.

King Maha Vajiralongkorn thanked those involved in finding the soccer team nicknamed the Wild Boars.

"This circumstance has clearly shown the power of unity in action, power of love and goodwill towards fellow men regardless of race and religious beliefs," the king said in a letter.

Some relatives of the boys gathered early on Thursday near the cave, where only a few rescuers were marching up to its entrance, in contrast to days of frantic activity during a search that has grabbed media attention around the world.

One mother said she had not been able to contact her boy.

"We can't send them messages yet," said Ratdao Chantrapul, 37, the mother of 14-year-old Prajak Sutham.

"Yesterday, they tried to take in mobile phones but the bag it was in broke," she said.

Volunteers have been descending on the cave site to join the multinational rescue team, which includes Australian police, US military personnel and British cavers as well as more than 1,000 Thai army and navy personnel.

Father and son team Rafael and Shlomi Aroush drove for 12 hours from their home in Udon Thani province to help with the rescue effort, arriving at the cave early on Thursday.

Rafael, 53, who is from Israel but has lived in Thailand for more than 30 years, said he had been inside the cave 25 years ago and found that it was more difficult to navigate than other caves he had tackled.

"It's very narrow. The way divers have to work is not a normal way of diving. The visibility is also really bad. You're diving blindly," Aroush told Reuters.

He said it would be challenging for the boys.

"Learning to dive is easy, but this is not normal diving," he said.

Reuters