CHIANG RAI: Navy Seal divers in flooded Tham Luang cave were on Monday afternoon within 500 metres of a chamber containing an elevated rock mound, nicknamed "Pattaya Beach" by cavers, which could have provided a refuge to 12 boys and their football coach trapped in flooded Tham Luang cave for more than a week when heavy rains flooded it, blocking the way out.

Progress has been slowed as divers need to widen parts of a narrow 100 metre stretch -- with sharp bends -- that they were unable to pass through without their air cylinders becoming jammed.

Chiang Rai governor Narongsak Osotthanakorn earlier said rescuers had managed to reach the "Sam Yak" three-way junction - and now the most critical section of the murky underground labyrinth lies ahead.

"The Seal unit last night reached the T-junction and today they will press ahead to the left, but one obstacle we've found is a very small hole which we need to widen so that people can go through," the governor told reporters on Monday.

"This is today's focus. The passage must be penetrated. It is very narrow... They have not yet reached 'the (Pattaya) beach'," Mr Narongsak said.

Turning left from Sam Yak, the passage ascends, drops steeply and then bends to the left - and is not yet accessible to foreign divers and their equipment, the governor said.

'Pattaya Beach' is about two kilometres from the divers' forward command centre in the sprawling cave network's third chamber --  a total of roughly five kilometres from the main entrance of the cave in the Tham Luang Khun Nam Nang Nong forest park in Thailand's northernmost district of Mae Sai.

Deputy Prime Minister Prawit Wongsuwon said in Bangkok that rescuers hoped to reach 'Pattaya Beach' Monday -- but the strong water current in the narrow passage presents a major obstacle. 

All-out efforts continued to drain water from the cave and prevent fresh water from flowing into it, further raising the water level.

Above the cave, officials and volunteers were searching for shafts that might provide a back door into the cave.

"We are exploring 11 potential shafts today. The deepest one is 200 metres. If it goes down through, we will be very lucky," the Chiang Rai governor said.