A decision on a controversial plan to build a hydro-power dam on the Mekong River has been postponed pending further environmental studies.
The project at Xayaburi in Laos would be the first to be built on the mainstream of the lower Mekong.
Laos wants to sell most of the electricity to neighbouring Thailand.
But Cambodia and Vietnam say the dam will hit fish stocks and threaten the livelihoods of millions of people downstream who depend on the river.
The move came as representatives of all four countries met in the Cambodian city of Siem Reap.
The group, which collectively makes up the Mekong River Commission, last met in April but failed to agree on the project then.
Opponents fear the proposed $3.5bn (£2.15bn) Xayaburi dam and power station project would open the door to a building spree, degrading the Mekong’s fragile ecology and associated fishing industries.
Cambodia is planning mainstream Mekong dams of its own and many more have been proposed in Laos.
The group say they will now approach Japan to conduct a further environmental impact study.
“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.
