WASHINGTON (AP) -- The United States on Monday criticized a decision by the struggling Asian nation of Laos to build the first dam across the mainstream of the Mekong River, a project that environmentalists warn could affect tens of millions of livelihoods and trigger a dam-building spree along Southeast Asia's mightiest waterway.
The U.S. has urged a moratorium on such projects until impact studies are complete. But the State Department said Monday that Laos has announced its intention to start construction on the $3.5 billion Xayaburi dam despite lingering concerns downstream.
"The extent and severity of impacts from the Xayaburi dam on an ecosystem that provides food security and livelihoods for millions are still unknown," the department statement said.
Laos is one of Asia's poorest nations and hydropower is already a key source of revenue. The project, which will generate electricity for sale to neighboring Thailand, is strongly opposed by longtime Lao ally, Vietnam.
Opponents say the dam in central Laos would open the door for a building spree of as many as 10 other dams on the 3,000-mile river in Laos and Cambodia, degrading its fragile ecology and affecting the livelihoods of residents who rely on its fish and its water for irrigation.
The State Department said the U.S. has a strong interest in the sustainable management of the river and understands that members of the Mekong River Commission — a regulatory agency that includes representation from all four affected nations — has not yet reached consensus on whether the project should go ahead.
"We hope that the government of Laos will uphold its pledge to work with its neighbors in addressing remaining questions regarding Xayaburi," the statement said.
Vietnam, which has fraternal communist party relations with Laos dating back to the Vietnam War, has urged at least a 10-year moratorium on all mainstream dams on the Mekong.
A commission meeting in April deferred a decision on the dam, but that outcome is not binding. An approach road and other dam-related facilities to the Xayaburi dam are already being built.
Media reports Monday quoted Lao deputy energy minister Viraphonh Virawong as saying full construction would formally start Wednesday. He said that modifications had been made to the design of the dam to address environmental concerns.
Laos is currently hosting a meeting of Asian and European leaders. The Lao Embassy in Washington said Monday it had no information about the dam project.
The dam would cut across a stretch of the river flanked by forested hills, cliffs and hamlets where ethnic minority groups reside in Xayaburi province, forcing the relocation of about 2,100 villagers and impacting many more. Environmentalists say it would also disrupt fish migrations, block nutrients for downstream farming and even foul Vietnam's rice bowl by slowing the river's speed and allowing saltwater to creep into the Mekong River Delta.
China has placed three dams across the upper reaches of the Mekong and more are planned. But otherwise the mainstream flows free.