Published: Nov 05, 2012 9:39 PM EST
Updated: Nov 06, 2012 7:30 AM EST

WASHINGTON (AP) - The U.S. is criticizing a decision by the Asian nation of Laos to build the first dam across the mainstream of the Mekong River, which environmentalists say 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.

The State Department said Monday that Laos has announced its intention to start construction on the $3.5 billion dam despite lingering concerns downstream. The statement said the extent and severity of the dam's ecological impacts are still unknown.

Laos is one of Asia's poorest nations and hydropower is a key source of revenue. The project will generate electricity for sale to neighboring Thailand. Vietnam, a longtime ally of Laos, opposes the dam.