Egypt on Thursday rejected what it called Ethiopia's "unilateral" actions over its controversial Nile dam project, after Addis Ababa announced the multi-billion-dollar project was now complete and will be formally inaugurated in September.

The Grand Ethiopian Renaissance Dam (GERD) launched in 2011 with a $4-billion budget, is Africa's largest hydroelectric project, stretching 1.8 kilometres wide and rising 145 metres (475 feet) high.

Ethiopia views the dam as central to its development and electrification goals, but Egypt and Sudan – located downstream – have long raised concerns that it could severely impact their water supplies.

"Egypt categorically rejects any attempt to achieve development in Ethiopia at the expense of the rights of the downstream countries," Egypt's water resources minister, Hani Sewilam, said during a meeting with diplomats.

Ethiopia says dam complete

Sewilam accused Ethiopia of using "evasion and backtracking" in place of genuine negotiations.

Cairo has repeatedly warned that Ethiopia's actions violate international law and disregard years of talks aimed at securing an agreement on how the dam should be filled and operated.

In a speech to parliament, Ethiopian Prime Minister Abiy Ahmed said the dam was "now complete" and Addis Ababa was "preparing for its official inauguration."

"To our neighbours downstream – Egypt and Sudan – our message is clear: the Renaissance Dam is not a threat, but a shared opportunity," he said.

'Unilateral measures'

But Cairo and Khartoum remain unconvinced.

Egyptian President Abdel Fattah al-Sisi and Sudan's de facto leader Abdel Fattah al-Burhan met earlier this week and renewed their opposition to "any unilateral measures" on the Nile.

Egypt relies on the Nile for nearly all of its freshwater needs and fears that uncontrolled operation of the dam could drastically reduce its share of the river.