But the NCP and Sudan People's Liberation Movement (SPLM), which governs the south, cannot agree on who will be eligible to vote.

"The issue of the Abyei referendum has come to a standstill," Deng Arop, a SPLM representative who heads Abyei's administration, told reporters on Sunday.

"This has the potential to ... cause a regional and international conflict."

More than two decades of bitter war between north and south Sudan left an estimated two million people dead. A peace deal signed in 2005 created a federal unity government that shared power between the north's ruling party and the former southern rebels.

Tribe controversy

Abyei's referendum law gives the right of vote to members of the southern Dinka Ngok tribe and it is up to the referendum commission to decide which "other Sudanese" are considered residents of the region and therefore eligible to vote.

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The ruling NCP says the Misseriya, a big pro-unity nomadic tribe which grazes its cattle in the south during the dry season, should also vote.

The SPLM says the tribe as a bloc should not be allowed to vote, but that individuals with long-term residence in the region should be able to do so.

"The Misseriya ... are in no way meant to vote in the Abyei referendum because they are not residents. They are meant to be nomads," Arop said.

He said Misseriya had begun to settle 75,000 people in the north of Abyei to change the demographic of the region and influence the vote.

Arop estimated there were about 100,000 original Abyei residents excluding the Missiriya.

He called on the NCP to stop the settlements.

"If the government is not supporting this then it should take action to stop it," he said.

Abyei has been a contentious issue between the SPLM and the NCP both
before and after the 2005 peace deal.

Border arbitration

Deadly clashes between the Sudanese army and the SPLM in Abyei in May 2008 raised fears of a return to war between north and south Sudan. Both parties decided to take the matter of the sensitive border to arbitration in The Hague.

The Permanent Court of Arbitration refined the borders, leaving the Heglig oil fields in the north, out of the Abyei region.

Both north and south authorities have accepted the ruling, but it was criticised by the Misseriya tribe.

Douglas Johnson. a former former member of the Abyei Boundaries Commission, told Al Jazeera that the threat of renewed violence in Abyei is "very serious".

"There have been clashes on the border, there have been clashes within Abyei, and this latest report of movement in large scale of Misseriya into northern areas of is very worrying," he said.