Abyei, Sudan - The Ngok Dinka tribe is holding a controversial referendum on whether their oil-rich Abyei area should join South Sudan.
The quarrel over Abyei is one of the main unresolved disputes after the 2005 peace agreement between Sudan and South Sudan, which ended a bloody civil war and led to the independence of South Sudan.
Both the Sudan and South Sudan governments, along with the African Union that is playing a mediation role, do not recognise the referendum.
About 100,000 Ngok Dinka returned to Abyei for the referendum, which will be conducted over a three-day period with results announced on Thursday.
The region's majority Ngok Dinka people are believed to be in favour of joining South Sudan. The Sudan-allied Misseriya nomads, who come to Abyei to find pasture for their cattle, will not be allowed to participate in the referendum and have warned it cannot take place.
Clashes between both ethnic groups have occurred sporadically and flared dramatically in 2008 with hundreds of casualties.
There are concerns the referendum could stoke more violence.