CAIRO (AP) — Egypt, Ethiopia and Sudan resumed their years-long negotiations Sunday over the controversial dam Ethiopia is building on the Nile River’s main tributary, officials said.

The resumption of talks came after President Abdel Fattah el-Sissi and Ethiopia Prime Minister Abiy Ahmed said last month that they aim to reach within four months an agreement on the operation of the $4.6 billion Grand Ethiopian Renaissance Dam on the Blue Nile. The Blue Nile meets the White Nile in Sudan’s capital of Khartoum, before winding northward through Egypt to the Mediterranean Sea.

Egypt fears a devastating impact if the dam is operated without taking its needs into account. It called it an existential threat. The Arab world’s most populous country relies almost entirely on the Nile to supply water for agriculture and its more than 100 million people. About 85% of the river’s flow originates from Ethiopia.

The Egyptian Irrigation Ministry announced the new round of talks in Cairo. Irrigation Minister Hani Sewilam said Egypt wants a legally binding agreement on how the giant dam is operated and filled.

Sewilam said there are many “technical and legal solutions” for the dispute, without elaborating.

Tensions have heightened between Cairo and Addis Ababa after the Ethiopian government began filling the dam’s reservoir before reaching an agreement.

Key questions remain about how much water Ethiopia will release downstream if a multi-year drought occurs and how the three countries will resolve any future disputes. Ethiopia has rejected binding arbitration at the final stage of the project.

Ethiopia says the dam is essential, arguing that most of its people lack electricity.

Sudan wants Ethiopia to coordinate and share data on the dam’s operation to avoid flooding and protect its own power-generating dams on the Blue Nile, the main tributary of the Nile. The dam is located just 10 kilometers (6 miles) from the Sudanese border.