BANGKOK — Laos formally began building a controversial hydroelectric dam on the Mekong River on Wednesday, despite comments from the country’s prime minister that the project had been delayed.

“We held the groundbreaking ceremony today,” said Rewat Suwanakitti, the deputy managing director of Xayaburi Power, the company leading the project. “The Lao authorities told us that we could begin construction.”

The dam is the first of several planned for the river, and is being built despite concerns that the dam will irreparably harm fish stocks, which are an important food source for millions of people in Thailand, Laos, Cambodia and Vietnam.

Electricity produced by the Xayaburi dam, named for the surrounding province in Laos, will be exported to Thailand, and Laotian officials say they are counting on billions of dollars in revenue from the project.