Tajikistan on Saturday diverted the flow of a major river to start building the world’s tallest dam and the main element of the Rogun hydroelectric power plant, a $3.9 billion project which Dushanbe hopes will secure its energy independence.

The Central Asian nation which borders Afghanistan lacks hydrocarbon resources and relies heavily on hydroelectric power, although its neighbors downstream complain that this disrupts their traditional agricultural works.

President Imomali Rakhmon’s office said in a statement he attended Saturday’s ceremony at which explosions were used to block the main riverbed of the Vakhsh river, paving way for the construction of a 335-metre dam for which Italy’s Salini Impregilo this year won a $3.9 billion contract.

Uzbekistan, another former Soviet republic and Central Asia’s most populous nation of 30 million, has repeatedly urged Tajikistan not to build Rogun. In order to power the hydro plants, Tajikistan needs to accumulate water during the summer – when it is needed downstream for irrigation – and then release it in the winter, causing spring floods downstream.

Highlighting the poor state of Tajikistan’s existing energy infrastructure, mostly built in the Soviet era, a malfunction at the country’s biggest power plant, Nurek, left most of the country in complete darkness for several hours late on Friday.

The outage also affected production at Tajikistan Aluminium Company, one of the country’s main hard currency earners, the firm said on Saturday without disclosing any details.

In a speech broadcast by state television on Saturday, Rakhmon said upgrades at the Nurek power plant would require $700 million. The new power plant, Rogun, will start providing electric power in late 2018, Rakhmon said.