Addis Ababa says the Grand Ethiopian Renaissance Dam (GERD), a $4bn hydropower project, is crucial to powering its economic development, but Egypt and Sudan consider it a serious threat to their vital water supplies.
Long-running diplomatic efforts to resolve the dispute between the three countries have yielded little success.
Ethiopia sees the GERD as the centrepiece of its bid to become Africa's biggest power exporter, with a projected capacity of more than 6,000 megawatts.
"Prime minister Abiy Ahmed announced the successful completion of the GERD's third filling," the prime minister's office said on Twitter, adding that Ethiopia was working "to ensure the benefits of the lower basin countries".
Abiy held a ceremony officially turning on the dam's second turbine on Thursday, six months after the first came online. Photographs shared by Abiy's office showed him standing at a podium in front of a huge plume of water being discharged from the front of the dam.
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