Voters in southern Sudan have begun the registration process for next year's referendum in which the south will seek independence. 

In addition to the broader referendum, a small region called Abyei, which sits astride the oil-rich border between the south and north, will vote to decide which part it wants to join.

There are already fears that the sharing of oil wealth may create trouble between the north and the south.

The Unity State, which is also rich in oil reserves, borders northern Sudan, and is a potential flashpoint.

Haru Mutasa gained exclusive access to troops from the Sudan People's Liberation Army in Unity State and reports.