Tensions are rising between Egypt and Ethiopia as Addis Ababa moves closer to diverting water to a massive hydroelectric project on the Nile that has been a center of a decade-long dispute over who controls Africa’s longest river

Talks between officials from Egypt, Ethiopia, and Sudan hosted by the Democratic Republic of Congo ended without an agreement on Tuesday, sparking a new round of heated rhetoric between the two countries.

Egypt’s president on Wednesday hinted at the possibility of conflict with Ethiopia, but said he preferred cooperation on the issue.

“I say to our brothers in Ethiopia, don’t touch a drop of Egypt’s water, because all options are open,” President Abdel Fattah Al Sisi said at a conference in Cairo.

Addis Ababa began construction on the $4.8 billion Grand Ethiopian Renaissance Dam in 2011, part of what the government says is a critical development project that will bring electricity to tens of millions of people who currently rely on firewood as their main source of fuel.

Egypt regards the dam as a strategic threat that could siphon off critical water supplies from tens of millions of people. The majority of Egypt’s 100 million people live in a narrow ribbon of land along the river and in the Nile Delta region.