The leaders of Sudan and South Sudan have been urged to agree a peace deal to overcome a bitter dispute during talks in Ethiopia.
UN Secretary General Ban Ki-moon called for "co-operation and mutual development", and the US said Sudan should accept an African Union border plan already agreed by South Sudan.
Tensions over oil and borders have threatened to spark a new conflict.
The UN has threatened sanctions if Sunday's talks do not produce a deal.
The Security Council has called for an urgent agreement on a demilitarised border zone.
Oil wrangle
President Omar al-Bashir and his South Sudanese counterpart, Salva Kiir, are due to meet in the Ethiopian capital, Addis Ababa.
Last month, the two countries agreed to re-open their land border after a closure lasting more than a year.
But the border remains unmarked.
On independence, the South gained two-thirds of the region's oil while Sudan retained the processing and export facilities.
In January, the South shut down oil production, accusing Sudan of stealing its oil, and the two countries' economies have been damaged as a result.
The BBC's James Copnall in Addis Ababa says the list of unresolved issues is formidable.
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