Investigators believe the 13 people, all males from Chiang Rai, crawled into the Tham Luang Nang Non Cave through a narrow 15-metre long channel.
Rising waters then blocked the path, which is about as wide as a single person, leaving the group stuck inside the cave network, Kamolchai Kotcha, an official with Thailand's national parks authority, told CNN.
"We have been working almost 24 hours but there are many limitations, the caves are pitch dark and very low level of oxygen in some areas, and scattering rains just made our task more difficult," said Kotcha.
The group have been missing since Saturday afternoon, when a park officer spotted bicycles parked idly by the entrance to the cave, despite it being off-limits.
Northern Thailand is famous for its remote and untouched caves, which draw visitors from around the world.
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