Divers are waiting for the water level inside Tham Luang to drop before again trying to reach a high chamber that may be serving as a safe haven from the floodwaters for the missing 12 boys and their football coach.

"Divers are ready for another operation once the water level goes down," the Royal Thai Navy SEAL posted in a Facebook message around 7pm on Wednesday.

Floodwaters inside Tham Luang cave are blocking attempts to reach the teen footballers and their coach. They went inside the cave in Mae Sai district, Chiang Rai, on Saturday afternoon and have not returned.

High-pressure water pumps were installed at the cave, draining 5,000 litres of water per minute.

The location in the cave rescuers want to reach, known as Pattaya beach, is an area of dry ground in a chamber deep inside Tham Luang.

Image made on Wednesday shows a topographical map annotated with location details of Tham Luang cave in which 12 young soccer team members and their coach went missing on Saturday. (Bangkok Post Graphics)

Anukoon Sorn-ek, who surveyed the cave, told radio station FM100.5 on Tuesday that it was a possible place for the trapped boys and trainer to have escaped the water flowing into the cave from outside.

Earlier reports had said the diving team had reached the area but Maj Gen Chalongchai Chaiyakham, the deputy commander of the Third Army Region, confirmed on Wednesday that they had not been able to get that far. He said the "beach" is about 5-6 kilometres from the entrance.

"Pattaya beach is a big chamber and there are also smaller chambers that could serve as a safe haven," he said. 

Long passages filled with murky water on the way into the mountain provide daunting obstacles for rescuers to overcome.

Other teams are surveying the mountain in search of shafts reaching down into Tham Luang as another point of access to the 13 trapped inside.

The search entered its fourth day on Wednesday and rescuers are working around the clock as time runs out.

Three British cave-divers, who have previously explored Tham Luang, arrived on Wednesday evening to help in the operation.