Egypt’s president says he expressed his “extreme concern” over the lack of progress in talks over the construction of a massive Nile dam in Ethiopia during a meeting with that country’s visiting prime minister.

Egypt fears the dam, which is about 60% complete, will significantly reduce its vital share of the Nile’s waters. Ethiopia has downplayed those fears and said it needs the dam for its own economic development.

A grim-faced President Abdel-Fattah al-Sisi emerged from a meeting in Cairo on Thursday with Ethiopian Prime Minister Hailemariam Desalegn, saying cooperation between Nile basin countries should not be a zero-sum game. Al-Sisi says Ethiopia has rejected an Egyptian proposal for World Bank experts to mediate the dispute.

Egypt depends on the Nile for almost all of its water needs.