The hydropower dam in San Sai in the south east of the country, sent five billion cubic metres of water gushing into six villages.
Fast flowing water has left areas submerged with boats drafted in for an emergency evacuation as water levels quickly rise.
The failure happened in the San Sai district in the southeastern province of Attapeu.
About 6,600 people have been left homeless after the floods.
Laos News Agency said “several human lives” were lost, adding “several hundreds of people are missing”, with many homes swept away.
Videos and images on social media show families standing on their roves as water rises all the way to the tops of houses.
Only lush trees and orange rooftops are visible above the brown muddy water.
Chaotic scenes show families wading through knee-high water carrying just one bag each. In one video an elderly lady is seen being carried to safety.
Other images show families packing onto boats with plastic bags of belongings.
Laos dam collapse: Hundreds are missing after the dam failure (Image: EXPRESS)
Families are trapped on rooftops after flood waters rushed in (Image: Attapeu Today Facebook)
Aerial images show vast areas now a sea of brown murky water.
An appeal for drinking water, food, clothing and medical supplies has been made.
A spokesman for SK Engineering & Construction, which is building the dam said: “We are running an emergency team and planning to help evacuate and rescue residents in villages near the dam.”
Prime Minister Thongloun Sisoulith has travelled to the area with officials to support the relief efforts, and has cancelled his meetings.
Laos dam collapse: Pictures show horror of Laos dam collapse
Tue, July 24, 2018
At least 100 people are missing after a hydropower dam in Laos collapsed
Attapeu Today
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Residents evacuate their village
There is no phone signal in the area and officials still don’t know how many people have died.
The dam holds about one billion tonnes of water. It collapsed after heavy monsoon rains, with first waters escaping at about 8pm last night local time (2pm BST).
It is not in operation yet, and was only due to begin generating electricity in 2019.
Just yesterday Xe-Pian Xe-namnoy, which owns the dam, issued a warning the dam was unsafe and it was planning to release 5,000 million litres of water, according to ABC Local.
Last year, another dam burst in the Xaysomboun province north of the capital Vientiane, which flooded surrounding villages.
The hydropower dam, built by Xe Pian-Xe Namnoy Power Company (PNPC), is the subject of controversy.
The communist country, one of the poorest in Asia, wants to become the “battery of Asia” and has embarked on an ambitious project to use hydropower dams to selling power to its neighbours.
A map of the Laos dam collapse site (Image: EXPRESS)
Villagers in the Attapeu area are being evacuated on boats (Image: Attapeu Today Facebook)
Environmental rights groups have for years raised concerns about Laos’ hydropower ambitions, including worries over the impact of dams on the Mekong River, its flora and fauna and the rural communities and local economies that depend on it.
The dam is the Xe-Pian Xe-namnoy Dam Impoundment, one of 11 signed off on the massive Mekong river which runs through the country.
Environmentalists have campaigned against the Laos government’s hugely ambitious hydroelectric projects.
The concerns of the green groups over the environmental impact of the projects were hardened into a full-on international dispute in 2012 when neighbouring Cambodia and Vietnam lodged fierce objections to the building of the Xayaburi dam which they said would severely impact fish stocks.
But Laos went ahead with the $100m dam – built by a Thai company with Thai money with almost all of the electricity pre-sold to Thailand.
It is due to send the majority of electricity, 90 percent, to Thailand, and the rest would be offered to Laos.
At the time Laos deputy energy minister Viraphonh Virawong said he hoped the Xayaburi dam would be the first of many and added: “I am very confident that we will not have any adverse impacts on the Mekong river.”
The Lao government is understood to have ambitious plans to use the damming of the Mekong to provide a massive eight percent of the entire electricity needs of south east Asia by 2025.
Laos dam collapse: People rescued by barge (Image: Pakaad.com)
A Laos village is submerged with flood waters from the dam (Image: ABC)
The water the dam is holding back (Image: FACEBOOK•MOHAMMAD MUHSIN)
Laos dam collapse: Xe-Pian Xe-namnoy Dam (Image: Facebook)
Although 11 dams form the backbone of the scheme Sang Lee, an employee of dam building firm SK Engineering & Construction, told the Phnom Penh Post more than 70 dams would be needed to complete the project.
The Lao government failed to release environmental impact documents leading to condemnation from across the world over the potential threat to people living downstream.
Although not officially confirmed it is widely believed up to 90 per cent of the power generated from the dams will not go to the underdeveloped, landlocked nation of Laos but will be sold across the border to the Electricity Generating Authority of Thailand.
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