Several people were killed and several hundred are missing after a dam in southeastern Laos collapsed, causing flash flooding in several downstream villages, Lao state-run media reported Tuesday.

The accident occurred late Monday at one of two dams of the under-construction Xe Pian-Xe Namnoy Hydroelectric Power Project in the Sanamxay district of Attapeu Province, according to the Lao News Agency, or KPL.

(Lao News Agency/Kyodo)

It said an auxiliary dam's collapse at 8 p.m. Monday released 5 billion cubic meters of water that caused flash flooding in the six villages of Yai Thae, Hinlad, Mai, Thasengchan, Tha Hin, and Samong, leaving more than 6,600 people homeless.

Provincial authorities called for emergency aid for the victims, including clothing, food items, drinking water, medicine and other relief items.

The 410-megawatt hydroelectric power project, valued at $1.02 billion, was being constructed by Xe Pian-Xe Namnoy Power Co., a joint-venture formed in March 2012 by four companies: SK Engineering and Construction and Korea Western Power, of South Korea; Ratchaburi Electricity Generating Holding PCL, of Thailand, and Lao Holding State Enterprise, of Laos.

Construction on the build-operate-transfer project began in February 2013 and commercial operation had been expected to begin in 2018.

Video footage posted on the Facebook page of ABC Laos news site showed people sitting on the rooftops of houses in their submerged village.

South Korea's Yonhap News Agency reported that SK E&C has formed a crisis management team to respond to the disaster, while the company's President Ahn Jae Hyun and senior company officials have left for Laos to assess the scope of flood damage and to support rescue operations there.

Although initial media reports said one of the dams under construction collapsed, the company said it is checking whether the dam actually failed or if water swept over it, according to Yonhap.

(Lao News Agency/Kyodo)

An SK E&C official was quoted as saying the area had been hit by heavy rainfall three times greater than normal. "The overflow of water went downstream, which had already been flooded, causing more damage," he said.

The report said helicopters were being brought in from Thailand by the company to help rescue stranded people. It noted that the affected area is surrounded by dense forest where communication is not easy, complicating rescue efforts.

According to the joint venture's website, the project foresees the construction of a series of dams, reservoirs and water transfer conduits to contain and divert waters of the Xe-Namnoy and Xe-Pian rivers in Attapeuto Province to a hydropower generating plant in Champasack Province.

Upon completion, the project is expected to generate 1,879 gigawatt hours per year, with 90 percent of the electric power it generates to be exported to Thailand and the remainder to be sold to the local grid.