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Southerners are widely expected to choose independence in the vote, due to take place from January 9, but Egypt's President Hosni Mubarak and Libya's Muammar Qaddafi have both called in the past for the nation to remain united.
The two leaders, both of whose countries share long, porous desert borders with Sudan, flew to Khartoum for more than two hours of talks with Sudanese President Omar Hassan al-Bashir and the leader of the semi-autonomous south, Salva Kiir.
After the meeting, the leaders issued a joint statement calling for a "peaceful, calm, transparent and credible environment that will reflect the will of southern Sudan's people," according to Sudan state radio.
They said they would respect the will of the southern Sudanese, whatever the outcome of the referendum.
U.S. President Barack Obama has written to Gaddafi, Mubarak and other leaders in the region stressing Washington's desire to see a peaceful and successful voting process.
North Sudan's ruling National Congress Party (NCP) and the main southern party, the Sudan People's Liberation Movement (SPLM), have been talking for months about the course of the north-south border, how to split oil income and other issues.
But there has been little public sign of progress. Each side has accused the other of building up troops, and analysts say disputes over preparations for the vote could reignite conflict.
The plebiscite was promised in a 2005 peace deal to end decades of civil war between the mostly Muslim north and the oil-producing south, where most people are Christian or follow traditional beliefs.
Egypt had proposed a looser confederation as an alternative to secession and its foreign minister said in November that it would not object to a referendum delay of several months.
Egypt is almost totally dependent on the Nile for its water, and is watching the vote closely for any effect on colonial-era pacts that give it most of the river's annual flow.
It has also aired worries that the referendum could trigger violence and an influx of Sudanese migrants into Egypt.
Qaddafi warned in October that southern secession could spur separatist movements across Africa and scare off investors.