US President Donald Trump told his Egyptian counterpart Abdul Fattah al-Sisi in a phone call on Tuesday that Washington will keep up “tireless efforts” for a deal between Egypt, Ethiopia and Sudan over an Ethiopia Nile dam, the Egyptian presidency said. The three countries had expected to sign an agreement in Washington last week on the filling and operation of the $4 billion Grand Ethiopian Renaissance Dam (GERD), but Ethiopia skipped the meeting and only Egypt has initialed the deal thus far. Ethiopia on Tuesday accused the United States of being "undiplomatic" in trying to resolve the row over the giant dam, but vowed to continue with ongoing talks. The GERD, set to become the largest hydropower plant in Africa, has been a source of tension between Addis Ababa and Cairo since Ethiopia launched work in 2011. The US Treasury Department stepped in last year to facilitate talks between Ethiopia, Egypt and Sudan -- another downstream country after Sisi reached out to Trump. Last week the Treasury Department issued a statement claiming an agreement had been reached and urged Ethiopia to sign "at the earliest possible time". Ethiopia, which skipped the most recent round of talks, denied there was a deal and expressed "disappointment" with the US statement, but did not spell out its position on future negotiations. At a press conference Tuesday, Ethiopian Foreign Minister Gedu Andargachew said Ethiopia would stick with the US-facilitated talks but warned Washington not to rush the process or try to influence the outcome. "The recent statement by the US we believe is undiplomatic and does not reflect a great nation like this," Gedu said. "We want Americans to play a constructive role. Any other role is unacceptable." Ethiopia sees the dam as essential for its electrification and development, while Egypt -- which depends on the Nile for 90 percent of its irrigation and drinking water -- sees it as an existential threat. The biggest initial hurdle is the filling of the dams enormous reservoir, which can hold 74 billion cubic meters of water. Egypt is worried Ethiopia will fill the reservoir too quickly, reducing water flow downstream. In its statement Friday, the Treasury Department said "final testing and filling should not take place without an agreement" -- a position endorsed by Sudan. But it is unclear when future talks might be held, and Ethiopia has said it will begin filling the reservoir "in parallel with" the dams construction.