With its rich grazing pastures, oil fields and history of tribal animosities, Sudan's disputed Abyei district could spark renewed border conflict after a referendum on its future status was shelved.

Here are some key facts on Abyei:

The 2005 peace accord that ended Sudan's devastating north-south civil war stipulated that two simultaneous referendums be held six years later, one on potential full independence for the south, and the other on whether Abyei lies in the north or the south.

But while the first of these is set to begin on Sunday, the second has been postponed indefinitely.

The former southern rebels, the Sudan People's Liberation Movement (SPLM) and their tribal allies, the Dinka Ngok, remain at loggerheads with the Misseriya pastoralists and the north's ruling National Congress Party (NCP) over who should be allowed to vote.

The flashpoint border region, with some of Sudan's biggest oilfields nearby, has long been a source of north-south tension.

After the peace deal was signed, Khartoum and Juba agreed to set up a commission to demarcate the borders of Abyei, which is the Dinka Ngok heartland, but its findings were contested.

In 2009, the two sides referred the issue to the Permanent Court of Arbitration based in The Hague, which reduced the region to an area of about 10,000 square kilometres (3,860 square miles) and left out the Heglig oil fields.

The ruling was welcomed by the SPLM, the Dinka Ngok and the NCP, which had secured its coveted oil fields. But it was rejected by the Misseriya, which fought for the north in the 22-year civil war.

The NCP subsequently reneged on its commitment and agreed to support the claims of their former civil war partners.

A tribe of Arab nomads, the Misseriya have long migrated to Abyei with their cattle during the dry season to take advantage of its grazing land and water from the Bahr al-Arab river -- or Kiir river to the southerners.

Under the referendum law, the Dinka Ngok are eligible to vote, but not the Misseriya, who fear they may lose access to the waters if Abyei joins the south.

With both sides heavily armed, renewed tensions could flare up in the area as they did in May 2008, when the worst fighting to rock the 2005 peace accord displaced 60,000 people and flattened Abyei town.

And renewed fighting broke out in Abyei between Misseriya and Dinka Ngok tribesmen on the eve of Sunday's southern referendum, leaving several people dead, according to a senior SPLM official.

Negotiations between the SPLM, the NCP, the Dinka Ngok and the Misseriya aimed at breaking the Abyei deadlock have yet to bear fruit.

Analysts warn the two sides are becoming radicalised and that the impasse has worrying implications for Sudan's other unresolved border disputes.

"The implications it has for the wider north-south boundary and the international border this might become are dire. This is why Abyei still matters," Douglas Johnson, an expert on southern Sudan, said in a study.

Sudan's north-south border runs to more than 2,000 kilometres (1,250 miles), but its demarcation is one of the key outstanding issues between the two sides, with some 20 percent yet to be agreed on.

The Misseriya, some of whom blocked a road used by southerners returning home in December and briefly kidnapped a Chinese oil worker, usually begin their seasonal migration to Abyei in early January.

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