Six environmental organisations opposing Laos' controversial Don Sahong hydropower dam yesterday took their complaints to a new level: The Human Rights Commission of Malaysia.

In a 23-page letter, the organisations from Cambodia, Thailand and the US collectively requested an investigation into the trans-boundary impacts of the 260-megawatt dam, which is being developed by Malaysian engineering firm Mega First Corporation Berhad.

"Unless the impacts on fish migrations are fully mitigated, and it is unclear if that is possible, the Don Sahong Dam can be expected to significantly reduce the number of migratory fish species that move between Vietnam, Cambodia, Laos and Thailand," Ian Baird, an expert in Southeast Asian geography and ecology, said in a statement yesterday.

The Don Sahong, which would lie less than two kilometres north of Cambodia, is the second of nine dams Laos plans to erect on the Mekong mainstream.

The developer's assessment of the dam's environmental impacts have been criticised by the World Wildlife Organization, among others, for lacking scientific baseline studies and for not considering its combined effect with multiple dams on downstream communities.

The developer's team has countered by saying that ongoing scientific monitoring began in 2009 and will continue 10 years after construction finishes.