At least 17 people have died after a dam collapsed in a remote part of land-locked Laos, local media reported.

More than 3,000 people need to be rescued after the collapse of the hydropower dam under construction in the southeast of the country on Monday.

Local media reports that about 2,850 people have been saved.

A wall of water was unleashed after parts of the dam were washed away, sending floods surging downstream, sweeping away homes.

"There are 17 bodies recovered so far, but we cannot estimate the number of missing yet," a Thai consular official said, adding that roof-level flood waters had submerged several villages near the Xe-Namnoy dam.

"All the dead are Laotian ... more than 6,000 are affected from eight villages near the dam," he said, from the relief centre in Attepeu province, where the disaster occurred.

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

Communist Laos has a vast network of rivers and scores of dams are being built or planned in the impoverished and landlocked country, which exports most of its hydropower energy to neighbouring countries such as Thailand.

The South Korean partner in the hydropower project said it discovered the upper part of the structure had washed away 24 hours before it collapsed.

SK Engineering & Construction said it discovered the damage to an auxiliary dam at around 9pm on Sunday local time.

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

President Michael D Higgins offered his sympathies to the country's president.

In a statement, he said: "As President of Ireland, I wish to offer the sympathy and the solidarity of the Irish people to the people of Laos as they come to terms with the loss of life and the hardship caused by the collapse of the dam in Attapeu province. 

"It was a privilege to meet with President Vorachith during my visit to his country in 2016, and I wish to reiterate Ireland's readiness to work with the people of Laos at this difficult time."