KHARTOUM, Jan 10 (Reuters) - Here are some facts about Sudan's disputed Abyei region which many fear could spark further north-south violence during and after the referendum on independence for southern Sudan.
Abyei sits on Sudan's ill-defined north-south border and is claimed by both halves of the country. In many ways it is a microcosm of all the conflicts that have split Sudan for decades -- an explosive mix of ethnic tension, ambiguous boundaries, oil and age-old suspicion and resentment.
Northerners and southerners fought hard over it during decades of civil war and have continued to clash there even after the 2005 peace deal that ended the war and set up the referendum.
Abyei contains rich pastureland, water and, after a recent re-drawing of its boundary, one significant oilfield -- Defra, part of a block run by the Greater Nile Petroleum Operating Company (GNPOC), a consortium led by China's CNPC.
It also has emotional, symbolic and strategic significance.
A number of leading figures from the south's dominant party the Sudan People's Liberation Movement (SPLM) hail from the area. Many southerners see the fight for Abyei as an emblem of their long struggle against perceived oppression.
For several months a year, Abyei is also used by Arab Misseriya nomads -- a well-armed group that provided proxy militias for Khartoum during the north-south war.
The Misseriya claim centuries-old rights to use the land for their livestock and Khartoum will have to back them to the hilt if it wants to keep them as allies. Abyei's Dinka Ngok tribe, with its ethnic links to the south, also claims its own historical ownership rights.
Abyei currently has a special administrative status, governed by an administration made up of officials from the SPLM and President Omar Hassan al-Bashir's northern National Congress Party (NCP). It is also watched over by Joint Integrated Units made up of northern and southern troops and police. In reality those units remain far from integrated and soldiers from both sides have been caught up in the fighting.
Abyei proved so intractable that it was left unresolved in the 2005 Comprehensive Peace Agreement that ended the north- south civil war.
Instead residents were promised their own Jan. 9 referendum on whether to join the north or the south. Plans for that vote have been left in limbo after a series of bitter disputes -- chiefly over the position of Abyei's borders and over who counted as Abyei residents with the right to vote.
The Permanent Court of Arbitration in The Hague came closest to solving the first in 2009 by re-drawing Abyei's boundaries, ceding several other key oilfields to the north. The SPLM and the NCP accepted the ruling but the Misseriya rejected it saying it still put too much of their pastureland inside Abyei. They have resisted official efforts to demarcate the new border.
The Dinka Ngok and Misseriya also remain at loggerheads over who gets to vote. The Dinka say only that a handful of settled Misseriya tradespeople count as residents. The Misseriya are demanding equal voting rights to the Dinka.
In the absence of a separate Abyei referendum, northern and southern leaders have promised to hammer out another settlement backed by mediation from the African Union and Washington. Various suggestions - including one that would cut Abyei in half, have been rejected outright by one side or the other.
Abyei residents threatened to hold their own referendum if the official plebiscite did not take place but that has so far not emerged.
WHY IT MATTERS
If north and south Sudan do eventually go back to war, it could easily be Abyei that sees the first fighting. Any return to north-south conflict would have a disastrous impact on the nine fragile countries that surround Sudan.
A settlement of Abyei would clear a huge stumbling block to Sudan's tortuous peace process and allow the sides to concentrate on equally explosive and unresolved issues, like how they would share out their oil revenues after a split. (Reporting by Andrew Heavens; editing by Myra MacDonald)