Cambodia, Laos, Thailand and Vietnam hold high-level talks on Thursday to decide whether to approve a controversial proposed dam on the Mekong River fiercely opposed by environmentalists.

The $3.8 billion Xayaburi project in Laos is the first of 11 dams planned for the mainstream lower Mekong, and activists warn that a green light could spell disaster for the roughly 60 million people who depend on the waterway.

Thailand, which has agreed to purchase some 95 percent of the electricity generated by the dam, has already indicated it will not oppose the project at this week's environment ministers' meeting in the Cambodian city of Siem Reap.

But Vietnam and Cambodia, wary of the dam's impact on their farm and fishing industries, have expressed strong concern and are calling for more studies on the impact of the vast 1,260 megawatt dam before it is allowed to go ahead.

Vietnam, voicing "deep" concerns about fish stocks and crucial sediment flows to the rice-growing Mekong river delta, has called for a 10-year moratorium on all hydro-electric projects on the lower Mekong.

The four member states of the intergovernmental Mekong River Commission have an agreement to cooperate on the sustainable development of the waterway and have been in consultations over the Xayaburi project.

In response to its neighbours' criticism of the project, Laos -- one of the poorest countries in the world which sees hydropower as vital to its future -- in May said it had suspended work on Xayaburi and commissioned a new review.

Last week, Laos indicated it should be allowed to go ahead, as "this dam will not impact countries in the lower Mekong River basin," deputy minister of energy and mines Viraphon Viravong told the official Vientiane Times.

Cambodia said this was not enough and called for more examination of cross-border impacts of the multi-billion-dollar project before a final decision is made.

"We will request Laos to carry out further studies," Te Navuth, secretary general of the Cambodia National Mekong Committee, told AFP Monday. "We don't understand everything about the project yet."

Environmentalists have warned that damming the main stream of the river would trap vital nutrients, increase algae growth and prevent dozens of species of migratory fish swimming upstream to spawning grounds.

"An immediate green light for Xayaburi equals taking an immense risk for the survival of several unique species," including the endangered giant Mekong catfish, conservation group WWF's technical expert Marc Goichot told AFP.

Major questions about the dam's impact, particularly on fish biodiversity and fisheries, have not been answered by Laos, he said.

Last week, US senators called for a decision on the dam to be delayed citing concerns over the "health and well-being of the more than 60 million people who depend on the Mekong River," Senator Jim Webb said in a statement.

Some 22,589 people from 106 countries have also submitted an international petition asking the ministers to cancel the project, according to environmental group International Rivers.

"The whole world is watching. We do not want to remember December 8 as the day the Mekong died," said Pianporn Deetes, Thailand coordinator for the group, which argues the dam is not needed to meet Thailand's future energy needs.

International Rivers has accused Laos of pushing ahead with construction of access roads to the site and work camps despite a lack of regional agreement.