Representatives from the four countries that share the lower reaches of the Mekong River said Tuesday that they failed to reach agreement on plans for a controversial dam in Laos. At a meeting in Vientiane, the Laotian capital, officials from Laos, Thailand, Cambodia and Vietnam postponed a decision on the Xayaburi dam, which would be the first across the Mekong south of the Chinese border. The dam is the first in a series of large-scale hydroelectric projects on the Mekong that the cash-strapped Laotian government is pursuing in order to sell electricity to other countries. Cambodia and Thailand recommended further studies of the dam’s environmental impact, and Vietnam, which fears deleterious effects to the Mekong Delta, called for a moratorium on dams across the Mekong. The governments did not say when they would meet to decide the issue.