ATTAPEU, Laos: The torrent of water unleashed in a deadly Laos dam collapse has drained into Cambodia, forcing thousands to evacuate, as rescuers on Thursday battled monsoon rains to find scores of Laotians still missing after entire villages were washed away.

Twenty-seven people have been confirmed dead, with 131 still missing, after the Xe-Namnoy dam collapsed on Monday in a remote southern corner of Laos, leaving villagers with little time to escape.

It is an unprecedented accident to strike the hydropower industry in Laos, where the Communist government has dammed large sections of its myriad waterways to generate electricity that is mostly sold to its neighbours.

The search and rescue effort entered a third day Thursday, with China, Vietnam and Thailand sending in specialists, while villagers picked through their wrecked, mud-caked homes for possessions as the flood waters receded.

Carcasses of livestock floated in the knee-deep waters in a devastated village visited by AFP, as heavy rain pounded the area.

Thousands of villagers downstream in Cambodia have also been forced to flee as the water once held back by the dam flowed south.

"Water is still rising, so more people will be evacuated," Men Kong, a government spokesman in Cambodia's Stung Streng province, told AFP.

Cambodian soldiers ferried villagers and motorbikes from flooded zones on wooden boats, while supplies were handed out to some who found refuge on dry land.

In Laos, Chinese rescuers in life jackets and helmets joined local soldiers searching for the missing Thursday, according to an AFP reporter at the scene, while community volunteers pitched in with private boats to return to villages still submerged.

Residents recalled their terror as water rushed through their homes.

Tran Van Bien, 47, from Ban Mai village close to the ruined dam said he was told to evacuate just two hours before the dam burst on Monday evening, running to a neighbour's house with his family as his home quickly filled with water.

"We were on the roof of that house the whole night, cold and scared. At 4:00 am a wooden boat passed and we decided to send my wife and my kid out," he told AFP from a nearby town where he eventually found dry land.

"My wife tied our child to her body, saying if they died, they would die together rather than being alone."

The $1.2-billion Xe-Namnoy dam, a joint venture between Laos, Thai and Korean companies, was still under construction in southern Attapeu province when it collapsed after heavy rains pounded the area earlier this week.

Two South Korean companies involved in the project's construction and operation said damage was reported a day before the auxiliary "Saddle D" dam collapsed.

However, a timeline from operator Korea Western Power Co obtained by AFP said 11 centimetres of subsidence was spotted at the dam's centre as early as Friday.

The company told AFP it could not yet determine the cause of the collapse.

"It is too early to define whether it was a natural disaster or a manmade disaster," a spokesman told AFP Thursday.

Thailand issued new regulations for its hydro plant operators in Laos this week, ordering companies to report on dam conditions every week and communicate closely with residents.

"If a dam plans to release water, they must coordinate with local officials to warn people and to prevent people from panic at least seven days [in advance]," Thailand's Energy and Mining Minister Khammany Inthirath announced Wednesday.

Southeastern Laos is frequently lashed by monsoon rains, and dam operators regularly release water from reservoirs in order to avoid overflow -- or collapse.

The 410MW Xe-Namnoy project is one of more than 50 hydropower plans underway in Laos, which has billed itself as the "battery of Asia" in its ambitious bid to become a major power exporter in the region.

Laos has said it wants to double its power generation capacity to 28,000 MW by 2020 and has opened its doors to foreign investors -- mainly from China, Thailand and Vietnam -- to build dams across the country.

But the projects have come under fire from rights groups who say local communities are forcibly moved and lose key access to river waters for farming and fishing.

"This tragedy has compounded their suffering," International Rivers said in a statement Thursday.

"Communities were not given sufficient advanced warning to ensure their safety and that of their families. This event raises major questions about dam standards and dam safety in Laos."