At a launch event in Prek Takeo, southeast of the capital Phnom Penh, Manet called the 180-kilometer (110-mile) project "historic", as fireworks shot into the air and drums sounded.

"We must build this canal at all costs," he said.

Once completed, the Funan Techo canal will run from a spot on the Mekong river, about an hour's drive southeast of Phnom Penh, to the sea in the Gulf of Thailand.

But the project comes shrouded in uncertainty, including its main purpose—whether for shipping or irrigation—who will fund it, and how it will affect the flow of the Mekong—one of the world's longest rivers.

Conservationists have long warned that the river, which supports up to a quarter of the world's freshwater fish catch and half of Vietnam's rice production, is at risk from infrastructure projects, pollution, sand mining, and climate change.

Cambodia, Laos, Vietnam, and Thailand are signatories to the 1995 Mekong River Agreement, which governs the distribution of the river's resources.

Cambodia has notified the Mekong River Commission (MRC) of its plans for the canal, but Vietnam wants more information about the project.