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.