The trapped Thai soccer team is being given a crash course in swimming and diving as rescuers rush to drain the flooded cave complex as much as they can before forecast heavy rain hits in coming days.

“What we worry about most is the weather,” Chiang Rai provincial Governor Narongsak Osatanakorn said on Thursday, adding heavy rain is predicted from Saturday.

“We can’t risk having the [water] flood back into the cave.”

Among those calling for a speedy rescue is Thailand’s king.

“Instructions were given by the King that everyone [rescuers] must bring out the children as quickly as possible,” Thailand’s Deputy Prime Minister, Prawit Wongsuwan, told The Bangkok Post on Thursday.

“If the water recedes and currents become less strong, they would be able to venture out. All parties are working their best, particularly the [Navy] Seals.”

The 12 teenagers and their soccer coach were found deep within the Thai Tham Luang cave complex on Monday. They had already spent nine days underground at the end of flooded and narrow passageways.

In video released on Wednesday, the boys were shown to be healthy and in good spirits. But getting them out is a dilemma for rescuers, who include experts from across the world.

Divers and medics have taken about four hours to reach the boys and their coach. Getting them out will likely take even longer, as none can dive and most cannot swim.

“We are talking kilometres of transport under the water with zero visibility,” said Claus Rasmusen, a certified cave diving instructor based in Thailand who has been helping the Thai SEAL team with logistics.

“It’s difficult.”

But there were glimmers of fresh hope on Thursday.

Fairfax Media reported that Poonsak Woongsatngiem, from the Department of Disaster Prevention and Mitigation in Bangkok, had said water levels had fallen about 40 centimetres at the mouth of cave complex thanks to intense pumping.

Rescuers are now able to walk as far as the third chamber, about 1500 metres in.

“We cannot say whether there will be a rescue today or not,” he said.

“The water level between the third chamber and the boys is still high. We put the tubes at the third chamber, trying to get the water out as much as possible.

“We hope that today or tomorrow the water level between the third chamber and the point that the students are at will decrease more.”

Despite constant efforts, water still reaches the cave ceiling in some deeper sections. Pumping is being stepped up even further, in a bid to give the group some headroom to pass through.

A foreign diver arrives to help with the rescue effort. Photo: Getty

CNN reported that Mr Rasmusen had said the boys had told divers they heard dogs barking, a rooster crowing and children playing while they have been underground.

That information has spurred teams into a renewed search for a chimney or hole they can use to get the teenagers out of the cave.

Mr Narongsak said the boys might be rescued in stages, depending on their physical condition.

“All 13 don’t have to come out at the same time,” he said. “Who is ready first can go first.”

He said all of the group had been practising wearing diving masks and breathing, but had not yet attempted any actual dives.

Wednesday’s video boosted the spirits of the families waiting above ground. Rescuers are also working to install an internet cable that would allow them to talk to the children.

Classmates of the trapped boys turned up at the site on Thursday to show their support for the group and the rescuers.

“We are here to pray and sing for them,” one of the children, Weerapat Kiat-orarnkul, said.

Another classmate, Piwit Akiriwong, said he wants “to play football with my friends again at school”.

A dozen Australian defence and police specialists boosted the hundreds of experts at the cave site in northern Thailand on Thursday. Foreign Minister Julie Bishop said the rescue effort had reached a critical stage.

-with agencies