A dam which Laos plans to build across the Mekong River could wipe out critically endangered dolphins in downstream Cambodia, the World Wildlife Fund (WWF) warned Thursday.

If built, the controversial Don Sahong dam in southern Laos would be just a kilometre upstream of the main stretch of water favoured by the rare freshwater Irrawaddy dolphin, WWF said in a statement.

With an estimated 85 adults, Cambodia hosts one of the largest populations of the species -- which is recognisable by its bulging forehead and distinctive blunt beak.

Conservationists have called for the suspension of the hydropower scheme, saying any restriction of the waterway will curb the number of fish swimming downstream and cut off the dolphins' main food source.

"If the Don Sahong dam is built, it will lead to the extinction of Mekong (Irrawaddy) dolphins," Chhith Sam Ath, WWF-Cambodia's country director, told reporters.

"Dolphins are very important for Cambodia. They attract tourists, they are a national treasure."

The dolphins face several other threats from the dam, the conservation group added, explaining explosives used in its construction could damage their highly sensitive hearing which they use to navigate.

The likely worsening in water quality, an increase in boats using the river and wider habitat degradation would further pressure the population, WWF said in a statement

Landlocked communist Laos agreed in January to hold further consultations with neighbouring countries before it starts building the dam.

Apart from the threat to the dolphins, wider fears for the livelihoods of tens of thousands of downstream fishermen and warnings of major environmental damage have also been raised.

WWF has urged Cambodian Prime Minister Hun Sen to call for a moratorium on the dam at the Mekong River Commission's summit in April.

The Cambodian government in 2012 approved a dolphin protection zone in a 180-kilometre-long (110 mile) river stretch from eastern Kratie to the border with Laos.

Freshwater Irrawaddy dolphins exist in three river systems in Southeast Asia, with Cambodia hosting the largest population.