Ethiopia has completed filling the reservoir of its huge dam on the Blue Nile river for a second year and the plant may start generating power in the next few months, a minister said on Monday, a move that has already angered Egypt and Sudan.

Addis Ababa says the Grand Ethiopian Renaissance Dam (GERD), a $4-billion hydropower project, is crucial to its economic development and to provide power.

But it has caused concern over water shortages and safety in Egypt and Sudan, which also depend on the Nile's waters.

"The second filling of the Renaissance dam has been completed and the water is overflowing," Seleshi Bekele, Ethiopia's minister for water, irrigation and energy, said on Monday in a Tweet.

"The next milestone for GERD construction is to realize the early generation in the next few months," he said.

Sudan's irrigation ministry said in a statement it rejected "unilateral measures from neighbouring Ethiopia and policies of imposing a fait accompli and ignoring the legitimate interests and serious concerns of its river partners".

Khartoum urged Addis Ababa to "continue negotiations...to reach a binding and comprehensive legal agreement that preserves the interests of all parties".

Egypt said last month it had received official notice from Ethiopia that it had begun filling the reservoir for a second time and said it rejected the move.

Egypt views it as a grave threat to its Nile water supplies, on which it is almost entirely dependent. Sudan has also expressed concern about the dam's safety and the impact on power generation from its own dams and water stations.

Long-running diplomatic efforts to resolve the dispute between the three countries have yielded little success.

The United States has also said Ethiopia's filling of the dam had the potential to raise tensions and has urged all parties to refrain from any unilateral actions.