Myanmar and Thailand have begun building a controversial hydro-power dam on the Salween River, the as yet longest undammed waterway in Southeast Asia, official media reported Thursday.

Thailand’s MDX Group Co Ltd has invested about six billion dollars in the Tasang project in eastern Shan State, which is the biggest of four planned dams on the Salween.

The state-run New Light of Myanmar newspaper said construction began on March 30 — a move likely to dismay environmentalists who had called for a delay to allow a public consultation on the project.

Activists say the dams could prove disastrous to Salween’s delicate ecosystem and...