BANGKOK — Hundreds of people are missing and an unknown number believed dead after the collapse of a hydropower dam under construction in southeast Laos, state media reported on Tuesday.

Laos is traversed by a vast network of rivers and there are several dams being built or are planned in the impoverished and landlocked country, which exports most of its hydropower energy to neighbouring countries like Thailand.

Laos News Agency said the accident happened at a hydropower dam in southeastern Attapeu province’s Sanamxay district late on Monday, releasing five billion cubic metres of water -- more than two million Olympic swimming pools.

The report added that there were "several human lives claimed, and several hundreds of people missing".

Several houses in the southern part of the district were also swept away, the report said, and officials in the province put out a call for relief aid for flood victims.

The US$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.

Among the companies involved in the project according to the Laos News Agency are Thailand’s Ratchaburi Electricity Generating Holding, South Korea’s Korea Western Power and the state-run Lao Holding State Enterprise.

The 410 megawatt capacity dam was supposed to start commercial operations by 2019, according to the venture’s website.

The project consists of a series of dams over the Houay Makchanh, the Xe-Namnoy and the Xe-Pian rivers in neighbouring Champasack Province.

It 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.

Under the terms of construction, PNPC said it would operate and manage the power project for 27-years after commercial operations began.

When contacted by phone in Laos’ capital Vientiane an official said: "We do not have any official reports about it yet. We are gathering information."

Dam projects, mainly providing power to neighbouring countries, have long been controversial as they often displace large numbers of local people.

The hydroelectric project at Xayaburi, led by Thai group CH Karnchang, with a 1,285 megawatt dam -- which will cost $3.5 billion according to state media -- is at the heart of Laos’ plan to become "the battery of Southeast Asia".

But it has sharply divided several Mekong nations who worry it will disrupt vital ecosystems and their own river systems.

Six Vietnamese workers were killed when a gas cylinder exploded at the construction site of a hydropower plant in central Laos in July last year. — AFP