Addis Ababa - African Union chief Nkosazana Dlamini-Zuma warned on Monday that a vote in the flashpoint Abyei region claimed by both Sudan and South Sudan was illegal and risked sparking a return to war.
Dlamini-Zuma said the "unilateral act is unacceptable and irresponsible" in a statement that "condemns this exercise in the strongest terms possible".
Patrolled by some 4 000 Ethiopian-led UN peacekeepers, the area is home to the settled Ngok Dinka, closely connected to South Sudan, as well the semi-nomadic Arab Misseriya, who traditionally move back and forth from Sudan grazing their cattle.
Luka Biong, spokesperson for the Abyei Referendum High Committee, which is organising the vote for the Ngok Dinka, said Monday that the majority of 65 000 registered voters have already cast their ballot in the three-day vote that began on Sunday.
Abyei was meant to vote on whether to be part of Sudan or South Sudan in January 2011 - the same day Juba voted overwhelmingly to split from the north - as part of the 2005 peace deal which ended Sudan's civil war.
That referendum was repeatedly stalled, and Sudanese troops stormed the Lebanon-sized enclave in May 2011 forcing over 100 000 to flee southwards, leaving a year later after international pressure.
Dlamini-Zuma, while saying she understood the "frustrations" arising from the long-stalled vote, "calls on those orchestrating this illegal act to put an immediate halt to their actions", the AU statement added.
"They pose a threat to peace in the Abyei Area, and have the potential to trigger an unprecedented escalation on the ground, which could negatively affect the continuing normalisation of relations between Sudan and South Sudan, with far-reaching consequences for the region as a whole," it added.
Only the Ngok Dinka are voting in the referendum - although organisers insist it is open to all - and the Misseriya have already angrily said they will not recognise the results of any unilateral poll.
Both Khartoum and Juba have criticised the vote, with South Sudanese government spokesperson Michael Makuei telling AFP it was "not a party to it" as it wanted a referendum "run according to the agreed system".