Ethiopia said on Sunday it had completed the fourth and final phase of filling a reservoir for its planned massive hydroelectric power plant on the Blue Nile, a project that Egypt and Sudan have long opposed.

Construction of the $4 billion Grand Ethiopian Renaissance Dam (GERD) began in 2011 and Ethiopia sees the project as crucial to powering its economic development.

Egypt and Sudan, however, consider the project a serious threat to their vital water supplies.

"Congratulations to all on the fourth filling of the Grand Ethiopian Renaissance Dam. Our national perseverance against all odds has delivered," Prime Minister Abiy Ahmed's office wrote on the social media platform X on Sunday.

With a projected capacity of more than 6,000 megawatts, Ethiopia sees GERD as the centrepiece of its bid to become Africa's biggest power exporter.

The three countries have been in protracted negotiations over the project.

In a sign of a potential breakthrough in July, Egyptian President Abdel Fattah al-Sisi and Abiy agreed on plans to finalise an agreement between the three countries on the filling of the dam and the rules for its operation.

But on Sunday Egypt's foreign ministry said Ethiopia's step "places a burden on the course of the resumed negotiations, the next round of which ... is hoped will witness a tangible and real breakthrough."