(Image source from: news.sky.com)

Hundreds of people are missing and an unknown number believed dead after a partially constructed hydropower dam in south-east Laos collapsed, sending flash floods surging through six villages, state media reported.

According to Laos News Agency, the mishap occurred on Monday evening at a dam in the country's far south, next to the border with Cambodia, releasing five billion cubic meters of water - more than two million Olympic-sized swimming pools.

The agency said there were "several human lives claimed, and several hundreds of people missing" while some 6,600 people had been made homeless as authorities scrambled to evacuate villagers from the devastation.

The other video recording showed household waiting for a rescue on the rooftop of their house, with a nearby Buddhist place of worship partially drowned.

After about 24 hours the collapse of the dam, the local authorities said they were struggling to measure the level of the catastrophe.

"We do not have any information yet about any casualties or how many are missing," an official in Attapeu province, where much of the flooding occurred, said 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.

It has been confirmed by a Thai company, which has been involved in the hydropower project that an auxiliary dam of 770m-long utilized to divert river water had been unsuccessful after the dense downfall.

"The incident was caused by a continuous rainstorm which caused a high volume of water to flow into the project's reservoir," Ratchaburi Electricity Generating Holding said in an English language statement.

The US$1.2bil (RM4.9bil) dam is portion of a project by Vientiane-based Xe Pian Xe Namnoy Power Company (PNPC) a joint venture formed in 2012.

The state-run Lao Holding State Enterprise and South Korea's Korea Western Power are as well involve¬d in the joint venture.

By Sowmya Sangam