Nineteen people died and more than 3,000 need to be rescued after a dam collapsed in a remote part of land-locked Laos, local media reported on Wednesday.
The English-language Vientiane Times, citing district governor Bounhom Phommasane, said more than 3,000 people "require rescue" and about 2,851 people have been saved.
About 19 people have been "found dead", the paper said. Rescuers were searching Wednesday for villagers left missing.
SK Engineering & Construction, one of two South Korean partners in the project, said in a statement late Tuesday that the top of one of five auxiliary earth-fill dams at the project "got swept away'' on Sunday night after heavy rains.
But AP reported Korea Western Power, which was due to operate the hydroelectric power plant after its completion, said problems first emerged Friday when workers discovered the dam's centre had sunk by 11 centimetres.
Updated map of Laos locating a dam collapse.
Repair work for the earth-fill dam was hampered by heavy rain, and damage worsened on Monday, causing water to overflow and flood seven out of 12 villages in the area, SK E&C said. It was helping to evacuate and rescue residents while also trying to contain further damage.
The cascade of 5 billion cubic metres from the collapse left more than 6,600 people homeless, the official Lao news agency KPL said.
The number of victims was unclear. KPL had said Tuesday afternoon that hundreds of people were missing, without providing details. More recent reports put the number missing in the dozens.
The government declared the area a disaster zone and top officials were rushing to the site, Vientiane Times said.
Photos and videos posted on social media showed people sitting on rooftops to escape the surging water, while others were carried to safety or rescued by boat. State media said helicopters were also being used to rescue people.
Continued heavy rain and strong winds forecast for the area could hinder rescue efforts, and risks from flooding persisted in the mountainous region.
Hundreds of people are missing and 19 are confirmed dead after the Xe Pian-Xe Namnoy dam under construction in Laos' Attapeu province collapsed on Monday evening. (Reuters video)
Provincial authorities issued a call for emergency aid -- clothing, food, drinking water, medicine, cash and other items -- from the "party, government organisations, business community, officials, police and military forces and people of all strata''.
The International Red Cross said food was a concern because village food supplies were drenched in the flooding. It was arranging for water purification units to be sent to the area to ensure supplies of clean drinking water.
The presidential office in South Korea said President Moon Jae-in had ordered an emergency relief team to help with the disaster.
SK E&C sent its president to Laos and set up an emergency team in Seoul, South Korea's Yonhap News agency reported.
The $1.02-billion project encompassing several river basins in a remote corner of southeastern Laos is the first hydroelectric dam to be built by a South Korean company, and it was unclear how severe the damage would be to the overall plan. The dam was due to begin operating in 2019, with 90% of the power generated going to Thailand.
Shares of companies affiliated with SK E&C sank on Wednesday on concerns over potential costs for compensation and other financial setbacks from the disaster.
SK Discovery Co, SK E&C's second-largest shareholder with a 28.25% stake in the builder, tumbled more than 10% to trade at its lowest level in more than three years. SK Holdings Co, the biggest shareholder in SK E&C with a 44.48% stake, fell 3%.
Laos has dozens of hydroelectric projects under construction and plans for sales of power to neighbouring countries, now accounting for about a third of its exports, to grow substantially.
But dam building along the Mekong River and its tributaries, including those affected by this disaster, has raised concerns over environmental impact and other problems.
International Rivers, a non-governmental group generally critical of such projects, said the catastrophe showed the need to improve warning systems.
"With over 70 hydropower projects currently built, under construction and planned across Lao PDR -- most of them owned and operated by private companies -- authorities must immediately review how dams are being planned, designed and managed,'' the group said in a statement.
(Video YouTube/FRANCE 24 English)