BY JAMES HOOKWAY

BANGKOK—A contentious $3.5 billion dam project on the Mekong River was put on hold again Thursday as nations called for further study of the environmental effects, a setback for Laos's plan to reinvent itself as the hydropower battery of Southeast Asia.

The Mekong River Commission, comprising Cambodia, Laos, Thailand and Vietnam, said in a statement that member governments had agreed to study the implications on the Xayaburi dam project further before giving Laos the go-ahead to continue construction—the second delay to the project this year.

The commission didn't say how long the study would take. "Further study will provide a ...