The framework also includes commitments to allow Sudanese citizens the right to live in either north or south Sudan. Observers say the move will reduce some tension ahead of January's referendum, for which voter registration has begun.

The referendum is part of a peace deal which ended the north-south civil war.

The AU statement said both parties "committed themselves to the immediate demarcation of the north-south border".

"In the event of secession, this will be the longest inter-state border in Africa, hosting in its immediate vicinity on both sides a significant proportion of Sudan's population," the statement said.

"The parties have committed themselves to maintaining a 'soft border', which will permit unhindered economic and social activity and interaction, which will be essential for economic prosperity and harmony between the north and the south."

Under the new framework, the disputed border area of Abyei will be addressed directly by the presidents of Sudan and the semi-autonomous south, the statement said.