Egypt, Ethiopia and Sudan have failed to reach a final agreement on the filling up and operation of what will be Africa's biggest hydro-electric dam that is being built on River Nile.

Ethiopian Ambassador to US Fitsum Arega has tweeted that the latest round of talks held in Washington on Thursday by water ministers from the three countries ended without a deal.

The ambassador did not specify why no agreement was reached.

The three nations had last month agreed on an initial deal, following talks mediated by the US and World Bank, that the Grand Renaissance Dam should be filled in stages during the rainy season. But more negotiation was needed before the final agreement could be finalised this month.

Ethiopia, which is building the dam, wants to start generating electricity as soon as possible.

But Egypt is concerned about its water supplies if the dam is filled too fast.