Energy-starved Laos failed to win approval from Southeast Asian neighbours on Thursday for a proposed hydropower dam on the Mekong River that faces fierce opposition from conservationists.

Ministers from Cambodia, Thailand, Vietnam and Laos postponed a decision on the $3.8 billion Xayaburi dam after meeting in the Cambodian city of Siem Reap, saying more research was needed into the likely effects of such projects.

"There is a need for further study on the sustainable development and management of the Mekong River including impact from mainstream hydropower development projects," they said in a statement.

The decision was welcomed by activists who warn that the vast 1,260 megawatt dam in Laos, the first of 11 planned for the mainstream lower Mekong, could spell disaster for the roughly 60 million people who depend on the waterway.

The four member states of the intergovernmental Mekong River Commission (MRC) have an agreement to cooperate on the sustainable development of the waterway.

Their announcement, which is not legally binding, added that the ministers agreed in principle to approach Japan and other development partners to support such studies.

Laos is one of the poorest nations in the world and sees hydropower as vital to its potential future as the "battery of Southeast Asia", selling electricity to its more industrialised neighbours Vietnam and Thailand.

Thailand, which has agreed to purchase some 95 percent of the electricity generated by the dam, had already indicated that it would not oppose the project at Thursday's meeting.

But Vietnam and Cambodia, wary of the dam's impact on their farming and fishing industries, expressed strong concern ahead of the talks and called for further studies to be carried out before it is allowed to go ahead.

Last week, Laos indicated it should get the green light, 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 further examination of cross-border impacts of the multi-billion-dollar project before a final decision is made.

Vietnam has even proposed a 10-year moratorium on all hydro-electric projects on the lower Mekong.

MRC chief Hans Guttman said the Xayaburi dam had not been specifically discussed on Thursday, with the focus instead on working together to study the impact of hydropower on areas such as fisheries and water quality.

"There's obviously a need for further work," he told AFP.

The 4,800 kilometre (3,000 mile) long river, the longest in Southeast Asia, is home to more than 700 species of freshwater fish including the endangered giant Mekong catfish, according to conservation group WWF.

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

Conservation group International Rivers said more than 22,000 people from 106 countries submitted a petition ahead of the meeting, asking ministers to cancel the project.

"Today the Mekong governments responded to the will of the people of the region," said Ame Trandem, Southeast Asia programme director for the group, which says the dam is not needed for Thailand's future energy needs.

"We welcome the recognition that not nearly enough is known about the impacts of mainstream dams to be able to make a decision about the Xayaburi Dam," she said.