A referendum in south Sudan on possible independence cannot he held as planned in January unless the border between north and south is demarcated in advance, a senior official of Sudan's ruling party said on Thursday.

"That is a condition for all referendums around the world that might lead to the creation of a new sovereign state," Ibrahim Ghandour, a senior official of the National Congress Party (NCP), told AFP.

South Sudan is still recovering from a 22-year civil war with the north during which about two million people were killed, in a conflict fuelled by religion, ethnicity, ideology and resources like oil.

The referendum was stipulated under a 2005 peace deal, which promised the south the chance to choose independence or to remain within a united Sudan.

And Ghandour added that failure to agree on a border could lead to a new conflict.

But a representative of the former southern rebels, the Sudan Peoples’ Liberation Movement (SPLM), was more circumspect.

Luka Biong Denk told AFP that "it is in the south's interest, even more so than that of the north, to demarcate the border," and that this would be preferable before the referendum.

"But we must not confuse what is desirable with what is required," underlining that it is not necessary to resolve the thorny border question before the vote.

As matters stand, the NCP and the SPLM are at loggerheads over five distinct sites along a tentative border, but Ghandour said "we have good hopes of reaching a deal before the referendum."