Sudanese Prime Minister Abdalla Hamdok on Monday invited the United Nations and Washington to mediate in Sudan and Egypt's dispute with Ethiopia over the upstream country's vast dam on the Nile.

"The prime minister sent letters asking the UN and the US to intervene and mediate in the dispute over the Renaissance Dam," said Faisal Mohammad Saleh, a spokesman for Hamdok.

The three countries have been locked for almost a decade in inconclusive talks over the filling and operation of the Grand Ethiopian Renaissance Dam (GERD) on the Blue Nile, which broke ground in 2011. 

Sudan last month called for mediation by a quartet of the UN, the US, the African Union and the European Union, a proposal welcomed by Cairo but rejected by Addis Ababa. 

Egypt sees the dam a threat to its water supply, while Sudan fears its own dams will be compromised if Ethiopia proceeds with filling the GERD before a deal is reached.

The two downstream nations have urged Ethiopia to hold off on filling the reservoir until a deal is signed. But Addis Ababa announced last July that it had completed its first year filling target and would proceed with the second stage in July this year, regardless of whether an agreement is in place. 

Last Thursday, Hamdok visited Egyptian President Abdel Fattah al-Sisi in Cairo, where they agreed to step up their coordination to reach a deal and voiced support for the quartet proposal.