Hundreds of people are missing and many others feared dead after an under-construction hydroelectic dam collapsed in southern Laos, state media reported on Tuesday. The collapse caused flash floods through six villages, said BBC, leaving more than 6,600 people homeless.

Prime Minister Thongloun Sisoulith called off government meetings and led ministers to monitor rescue and relief efforts in one of the affected areas, reported Lao News agency.

The Xe-pian Xe-Namnoy dam in San Sei district of Attapeu province collapsed at 8 pm on Monday, releasing 5 billion cubic metres of water, said Lao News. Construction started in 2013 and the 410 megawatt-capacity dam was due to start operating next year.

The company building the dam said heavy rainfall and floods caused the collapse. “We are running an emergency team and planning to help evacuate and rescue residents in villages near the dam,” a spokesperson of SK Engineering & Construction told Reuters.

Water swept through the villages of Yai Thae, Hinlad, Mai, Thasengchan, Tha Hin, and Samong, reported The Guardian. Boats were deployed to evacuate the stranded and aerial footage showed the region under water, with only roofs of houses and the tops of trees visible.

Authorities had hoped the dam would become a lucrative source of income for communist Laos by exporting 90% of the energy to neighbouring countries. However, environmental activists had repeatedly voiced their concern about the government’s scheme of building dams across the country’s Mekong river and turn it into a hydroelectricity hub. Over the coming two decades, 11 hydroelectric dams on the Mekong, along with 120 tributary dams, have been planned.

The International Rivers group, which works to protect water bodies and the rights of communities dependent on them, said the collapse exposed the “major risks” in some dam designs that are “unable to cope with extreme weather conditions”. “Unpredictable and extreme weather events are becoming more frequent in Laos and the region due to climate change,” the group told Reuters. “This also shows the inadequacy of warning systems for the dam construction and operations.”

It said the warning had appeared to come very late and when it did, it was ineffective in ensuring that people could get out of harm’s way well in advance.