A wall of water was unleashed Monday after parts of the Xe-Namnoy dam were washed away, submerging several villages with hundreds of people feared missing.

A Thai consular official at the scene of the relief effort in Attepeu province said 17 bodies had been "recovered".

"But we cannot determine the number of missing yet," Chana Miencharoen said, adding roof-level flood waters were hampering rescue efforts.

Footage on Laos television showed people huddled on roofs as muddy water several metres high swirled menacingly close below them.

"All of the dead are Laotian... more than 6,000 people are affected from eight villages near the dam," Chana added.

Poor but blessed with abundant natural resources, Laos aims to become the "Battery of Asia" allowing dozens of foreign-funded dam projects across its network of rivers.

But fears over the environmental impact of projects, which export the majority of their electricity to neighbouring Thailand and China, go unheard inside the closed, tightly controlled Communist country.

The remote area is only accessible by helicopter and flat-bottomed boats, with roads badly damaged by the flash flooding or completely washed away.

Rescue officials in neighbouring Thailand were reportedly stuck at the border with Laos officials sluggish in allowing access into the country.

South Korea, which has firms involved in the project, was sending a relief team to the area, President Moon Jae-in's spokesman said Wednesday.

"Although we are still determining the cause of the dam accident, our government must actively take part in on-site relief efforts without delay as our companies were involved in the construction of the dam," Moon was quoted as saying.

- Questions over warning -

Questions emerged on Wednesday about the efficacy of the early warning system in place in the event of a dam breach, after two South Korean companies heavily involved in the $1 billion project said damage was reported at least a day before the calamity struck.

The South Korean partner in the hydropower project SK Engineering & Construction said it discovered the upper part of the structure had washed away at around 9:00 pm local time on Sunday.

"We immediately alerted the authorities and began evacuating (nearby) villagers downstream," it said in a statement.

Repair work was hampered by heavy rain which had damaged roads, it said, and early on Monday water was discharged from the Xe-Namnoy dam -- one of the two main dams in the project -- to try to relieve pressure on the auxiliary structure.

The government was warned about further damage to the dam at around noon, prompting an official evacuation order for villagers downstream, and the structure collapsed a few hours later, it said.

Dam operator Korea Western Power Co. said one of the auxiliary dam -- "Saddle D" -- broke under heavy rain.

According to a timeline provided in a report to a South Korean lawmaker and obtained by AFP it said "11 centimetres of subsidence was found at the centre of the dam " as early as Friday.

Emergency repair equipment was unable to be used as the subsidence worsened.

"It remains unclear what caused the dam to subside in some places and develop cracks. But all of this happened under heavy rains," a Korea Western Power spokesman told AFP.

The $1.2 billion dam located near the border with Cambodia 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 the two South Korean companies, according to the project's website.

The 410 megawatt capacity plant 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.

It planned to export 90 percent of its electricity to energy-hungry Thailand and the remainder was to be offered up on the local grid.

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