If the Xayaburi is built, Hirsch adds, it will be easier for other lower Mekong countries, particularly Cambodia, to build dams on the Mekong's main stream.

Laos "cannot ignore" critical statements from its lower Mekong neighbors, according to a Vietnamese environmentalist who requested anonymity.

If Laos builds the dam over high-level opposition, the country may jeopardize its reputation in the international community, the environmentalist said.

Under MRC protocol, the four lower Mekong countries are required to notify their neighbors if they plan to build dams on the river's main stream, but they do not need each other's permission to proceed with dam projects.

China, which borders Laos, already operates four dams on the Mekong's upstream reaches.

Environmental impact

Civil society groups and non-governmental organizations across the region have warned that building dams on the lower Mekong will hurt the environment and threaten food security and rural livelihoods.

Stuart Chapman is conservation director for the World Wildlife Fund's Greater Mekong Program. He says the Xayaburi dam would have adverse effects on sediment flows and fish migration.

"The Mekong River is unique, both in terms of diversity of the fish and the volume of fish that travel up and down it," Chapman explained. "So essentially any dam that is built across the Mekong is going to be a blockage to many fish species as they try and migrate. If they can't migrate, they don't breed, and this will lead to a collapse of the fishery."