Sudan on Tuesday rejected an Ethiopian proposal to sign an initial agreement greenlighting the filling of a controversial mega-dam it is building.
"I cannot accept the signing of a draft agreement to the first phase (filling the dam) because it poses legal and technical problems that must be addressed," Sudanese Prime Minister Abdalla Hamdok said in a statement.
Ethiopian Prime Minister Abiy Ahmed had urged him to sign the agreement.
Both Khartoum and Cairo fear the dam will trap their essential water supplies once the giant reservoir starts being filled in July as planned by Addis Ababa.
Hamed Saleh, Sudan's chief negotiator in talks on the dam facilitated by the US administration, said "most of the issues at play... cannot be separated... including long-term environmental and social impacts".
Tensions have been high in the Nile basin ever since Ethiopia broke ground on the project in 2011.
Addis Ababa says the dam is crucial for its economy, while Egypt fears it will disrupt the river that provides almost all its water.
Sudan hopes the dam will provide much-needed electricity and help regulate flooding.
The US Treasury Department and the World Bank stepped in as observers last year to facilitate talks between the three countries after negotiations repeatedly failed.
"The only way to reach a comprehensive agreement is for all parties to go back to the negotiating table immediately," Hamdok added.
The 6,600-kilometer-long (3,900-mile) Nile is a lifeline supplying both water and electricity to the 10 countries it traverses.
Its main tributaries, the White and Blue Niles, converge in the Sudanese capital Khartoum before flowing north through Egypt to drain into the Mediterranean Sea.