Officials have brought boats to help evacuate people in San Sai district of Attapeu province in the southeast of the country as water levels rise after the dam collapsed, ABC Laos news reported.
It did not provide further details about the incident.
A video posted by the news network on its Facebook page showed villagers stopping to watch fast-flowing water from the side of a river bank.
Environmental rights groups have for years raised concerns about Laos' hydropower ambitions, including worries over the impact of dams on the Mekong River, its flora and fauna and the rural communities and local economies that depend on it.
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