The rescue of a baby boy, terrified and hungry after days without food, has been captured in a viral video showing the infant survivor of a dam collapse in southern Laos being carefully carried through swirling flood waters and waist-high mud.

Footage of volunteers from Thailand rescuing 14 people, including the baby, went viral when it was released on Friday as an increasingly international relief mission scrambles to save lives in a disaster that has left scores dead and missing.

The survivors were stranded by floodwaters after they fled up a hill on Monday as the Xe-Namnoy dam broke under heavy rain, leaving several villages devastated by flash floods.

The Thai team, who waded several kilometres through rushing water carrying uprooted trees and debris to rescue the group, are fresh from efforts to help free a youth football team trapped in a cave in the north of their country.

They have now come to help out in neighbouring Laos, which is poorly equipped to deal with natural disasters of this scale.

"The boy is four months old. He didn't have fever but he was crying, maybe because of the cold weather," Kengkard Bongkawong, one of the rescuers, who is from Thailand's north-east, told AFP.

"The baby was crying and looks terrified. Actually they were (all) still terrified of the rushing water."

The video had been watched nearly half a million times hours after it was posted online on Friday.

Earlier this week officials said 27 bodies had been retrieved so far, with the country's prime minister reporting 131 missing.

But on Friday the governor of Attapeu province Leth Xiayaphone revised down the toll to five, saying the larger number previously given was "unconfirmed information".

Secretive Communist authorities in Laos are not used to international scrutiny and have blocked access to foreign media, complicating efforts to establish the exact death toll.

The dam disaster has also raised serious questions over Laos' big bet on hydropower to propel it out of poverty.