Several people have been killed, hundreds remain missing and around 6,600 are homeless after a hydroelectric dam under construction collapsed in Laos, authorities said on Tuesday.

Officials did not specify the exact number of fatalities. The Xepian-Xe Namnoy dam, situated around 550 km southeast of the capital, collapsed on Monday, unleashing flash floods in six villages, Lao News Agency reported.

The administration of the province of Attapeu, where the dam was built, requested basic humanitarian assistance for the affected. Prime Minister Thongloun Sisoulith postponed government meetings and went to the affected area in Sanamxay district with senior officials to monitor relief efforts, the state media said.

Pictures showed villagers stranded on the roofs of submerged houses and boats carrying people to safety. “The disaster claimed several human lives (and) left hundreds of people missing,” according to the agency.

The Xe-Pian Xe-Namnoy Power Company (PNPC) began building the dam in 2013 and it was due to begin generating power this year.

Thai company Ratchaburi Electricity Generating Holding, a partner in the PNPC project, said that the collapse occurred due to continuous rain, which caused a high volume of water to flow into the project’s reservoir.

“Xe-Pian Xe-Namnoy Power Company Limited and related agencies had evacuated the people who reside around the area to temporary shelters (…). In addition, urgent assessment of the situation is being made in order to be able to immediately resolve the situation once the water level of the dam has decreased,” the company said in a statement.