Hundreds of people are missing and an unknown number feared dead after a partly built hydropower dam in southeast Laos collapsed after heavy rain, state media and contractors said on Tuesday (Jul 24).

The accident happened in southeastern Attapeu province late Monday, releasing five billion cubic metres of water - more than two million Olympic swimming pools.

Villagers take refuge on a rooftop above flood waters from a collapsed dam in the Attapeu district of southeastern Laos, Tuesday (Jul 24). (Photo: AP via Attapeu Today)

There were "several human lives claimed, and several hundreds of people missing", Laos News Agency said.

The incident triggered flash flooding that has affected nearly 7,000 residents in six villages, including Yai Thae, Hinlad, Mai, Thasengchan, Tha Hin and Samong, according to the news agency. More than 1,300 families have also been left homeless. ​​​​​​​

Aerial footage posted on the Facebook page of local news outlet ABC Laos showed a vast brown inundation swamping houses and jungle alike over a huge area.

This screen grab taken on handout video footage from ABC Laos on Jul 24, 2018 shows an aerial view of the flooded plains in Attapeu province after a dam collapsed the day before. (Photo: AFP/Handout/ABC Laos)

Nearly 24 hours after the collapse local authorities said they were struggling to gauge the extent of the disaster.

"We do not have any formal information yet about any casualties or how many are missing," an official in Attapeu province told AFP on condition of anonymity, adding that was no phone signal in the flooded region.

"We sent rescue teams who will help them and provide basic assistance first," the official added.

The Thai government has said it will send rescue experts to its northern neighbour.

Singapore has also offered to provide aid and immediate supplies to assist with flood relief efforts.

A video posted by the news network on its Facebook page showed villagers stopping to watch fast-flowing water from the side of a river bank.

Prime Minister Thongloun Sisoulith has travelled to Attapeu’s Sanamxay district to monitor the ongoing rescue and relief operation, Laos News Agency reported.

AUXILIARY DAM FAILED AFTER HEAVY RAIN

The USD 1.2 billion dam is part of a project by Vientiane-based Xe Pian Xe Namnoy Power Company, or PNPC, a joint venture formed in 2012 between a Laotian, a Thai and two South Korean companies, according to the project's website.

Ratchaburi Electricity Generating Holding, the Thai company, said it had been told by operators that a 770m-long auxiliary dam used to pert river water had failed after heavy rainfall.

"The incident was caused by continuous rainstorm which caused high volume of water to flow into the project's reservoir," it said in a statement.

One of the South Korean companies, SK Engineering and Construction, said it had sent a crisis team to Laos, according to Yonhap news agency, and was bringing in helicopters from Thailand.

Dogs take refuge on a rooftop above flood waters from a collapsed dam in the Attapeu district of southeastern Laos, Tuesday (Jul 24). (Photo: AP via Attapeu Today)

South Korea's foreign ministry said another Korean firm, Korea Western Power, was also involved. The companies and others had sent helicopters, boats and rescue workers.

"All of our 53 nationals who were taking part in this construction evacuated in advance," the ministry said in a statement.

The 410 megawatt capacity dam, which began construction in 2013, was supposed to start commercial operations by 2019, according to the venture's website.

The project planned to export 90 per cent of its electricity to energy-hungry Thailand and the remaining amount was to be offered up on the local grid./.

VNF/CNA