Marathon efforts to strike a deal between rival neighbours Sudan and South Sudan stretched late into the night Wednesday, as earlier optimism that an agreement would be signed appeared to fade.
Negotiations between former civil war foes President Omar al-Bashir of Sudan and his Southern counterpart Salva Kiir stretched for the fourth long day in the African Union-mediated talks.
Diplomats said the future of the contested flashpoint border region of Abyei -- a Lebanon-sized border area currently controlled by Ethiopian peacekeepers -- was a key sticking point.
Ethiopia's foreign ministry had initially called a press conference to witness the "final result of the negotiation between the two Sudans".
But a room specially prepared for a signing ceremony was closed Wednesday evening as talks ran on, and reporters waiting for the signing were sent away for the night.
The leaders returned to face-to-face talks Wednesday evening, the latest in a series of meetings since talks began Sunday, in what was originally billed as a one-day meeting.
Efforts had been made to strike a comprehensive deal tackling issues including oil, security, a demilitarised border buffer zone and contested regions, but it is understood that any deal -- if signed -- would be partial.
Amid international pressure to reach a deal -- after missing a UN Security Council deadline to settle by Saturday -- teams from each side have spent days locked in efforts to narrow positions, as mediators shuttle between them.
The protracted talks under AU mediation began in the Ethiopian capital several months before South Sudan split in July 2011 from what was Africa's biggest nation, following an independence vote after decades of war.
Key issues include the ownership of contested regions along their frontier, especially Abyei, and the setting up of a demilitarised border zone after fierce clashes earlier this year.
A comprehensive deal -- as opposed to another stepping-stone agreement -- would have to include a settlement on Abyei.
The buffer zone would also potentially cut support for rebel forces in Sudan's Southern Kordofan and Blue Nile regions that Khartoum accuses Juba of backing, just as the South accuses Sudan of arming rebels in its territory.
The UN set a deadline for a deal after border fighting broke out in March, when Southern troops briefly wrested the valuable Heglig oil field from Khartoum's control, and Sudan launched bombing raids in response.
Any deal would not include solutions to the growing humanitarian crises in Sudan's civil war states of Southern Kordofan and Blue Nile, which are not part of the direct talks.
However, it was hoped the deal would at least settle the details of last month's agreement to fix the oil export fees that landlocked South Sudan will pay to ship crude through Khartoum's pipelines to the Red Sea.
At independence, Juba took two-thirds of the region's oil, but processing and export facilities remained in Sudan. In January, the South shut off oil production after accusing Sudan of stealing its oil.