The planned 2.6-billion-euro hydropower project in northern Laos would have a capacity of 1,260 megawatts.
But, at the Vientiane meeting, Cambodian, Thai and Vietnamese officials from the neighbouring countries, which are also on the Mekong, said the environmental impact of Xayaburi have not been properly studied.
Vietnam in particular expressed "deep and serious concerns" about a lack of adequate assessments, calling for the deferment of planned hydropower projects on the mainstream Mekong for at least 10 years.
The dam, which is the first of 11 proposed on the mainstream lower Mekong, is now set to be discussed at the ministerial level.
The four member states of the Mekong River Commission, an intergovernmental body, have an agreement to cooperate on the sustainable development of the river and have been in consultations over the Xayaburi project.
But the final decision on the dam rests with Laos, which seems determined to press ahead, with state-run media insisting that the government "has full rights" to decide whether to approve construction of the dam.
"Project developers have begun to build roads to the project site, while the government is considering a concession agreement," the Vientiane Times reported on Tuesday. "Developers expect construction of the Xayaburi Mekong hydropower plant to begin in the near future and take eight years to complete."
The wildlife organisation WWF has warned that the Mekong giant catfish, one of the world's biggest freshwater fish, could be driven to extinction if plans to build hydropower dams on the river go ahead.