A last-ditch attempt to resolve a decade-long dispute between Egypt and Ethiopia over a huge new hydropower dam on the Nile has failed, raising the stakes in what – for all the public focus on technical issues – is a tussle for control over the region’s most important water source. The talks appear to have faltered over a recurring issue: Ethiopia’s refusal to accept a permanent, minimum volume of water that the Grand Ethiopian Renaissance Dam, or GERD, should release downstream in the event of severe drought.

What happens next remains uncertain. Both Ethiopia and Sudan – a mutual neighbor that took part in the talks – said that progress had been made and left the door open to further negotiation. Yet the stakes in a region acutely vulnerable to the impact of climate change are disconcertingly clear.

Ethiopia has threatened to start filling the dam’s reservoir when the rainy season begins in July, with or without a deal, a step Egypt considers both unacceptable and illegal. In a statement late Wednesday, Egypt’s irrigation ministry accused Ethiopia of refusing to accept any effective drought provision or legally binding commitments, or even to refer the talks to the three prime ministers in an effort to break the deadlock. Ethiopia was demanding “an absolute right” to build further dams behind the GERD, the ministry said.

Egypt’s Foreign Minister Sameh Shoukry threatened on Monday to call for United Nations Security Council intervention to protect “international peace and security” if no agreement was reached. A day later his Ethiopian opposite, Gedu Andargachew, accused Egypt of “acting as if it is the sole owner of the Nile waters.”