Xayaburi is the first of 11 hydropower dams -- nine in Laos and two in Cambodia -- proposed along the river.
About 95 percent of the dam's 1,260-megawatt capacity is intended for export to Thailand, which is financing the project. Thailand would operate the dam, turning it over to Laos after 30 years.
The Mekong River Commission's agreement Thursday to delay a decision on approval for the dam and to conduct further studies was reportedly already made by the prime ministers of MRC countries -- Cambodia, Laos, Thailand and Vietnam -- on the sidelines of the ASEAN meeting in Indonesia last month, International Rivers says.
MRC also delayed a decision on Xayaburi last April.
"Further study will provide a more complete picture for the four countries to be able to further discuss the development and management of their shared resources," Lim Kean Hor, chairman of the Mekong River Commission's council and Cambodia's minister for water resources said Thursday, The Wall Street Journal reports.
The longest river in Southeast Asia, the Mekong stretches 3,000 miles to the South China Sea and is home to more than 700 species of freshwater fish, including the endangered Mekong catfish. The Lower Mekong supports nearly 60 million people who depend on it for their livelihood, says the World Wildlife Fund.
Environmentalists warn that the proposed Xayaburi dam poses a threat to the environment and surrounding communities.
A study funded by the U.S. Agency for International Development that examined the impact of all the region's proposed dams found that the projects could lead to a net loss of up to $300 billion.
International Rivers says the $300 billion would represent lost income to the region because the fish stocks are a vital resource for the livelihoods of the people.
An international petition with more than 22,000 signatures calling for the project to be scrapped was submitted Nov. 30 to the prime ministers of Laos and Thailand.
"Ultimately the only responsible solution is to cancel the Xayaburi Dam and other dams planned for the Mekong River. We are confident that scientific studies on the Xayaburi Dam's impacts, conducted in a transparent, participatory, and independent manner, will reach the same conclusion," Teerapong Pomun, director of Thai environmental group Living River Siam, said in a statement.