Sudan's president arrived in the southern Sudanese capital Tuesday for talks with the leader of the semi-autonomous south, Salva Kiir.
The visit by President Omar al-Bashir precedes Sunday's referendum on whether the oil-rich south will secede, the BBC reported.
Officials said nearly 4 million people have registered to vote in the referendum, more than 95 percent of them in Southern Sudan.
The rest have signed up in northern part of the country and eight countries abroad.
The referendum is part of a 2005 peace deal that ended two decades of civil war.
For the vote to be considered valid, 60 percent of voters in Sudan must take part.
Chan Reec Madut of the Southern Sudan Referendum Commission said the vote wouldn't be affected by problems with access to polling stations in remote areas.