Counting of ballots has begun in an unofficial referedum held in the flashpoint region of Abyei to decide whether the oil-rich district should join Sudan or South Sudan.
Referendum organisers started the counting on Wednesday just as the Arabic-speaking Misseriya tribe, loyal to Sudan, vowed to hold their own referendum.
Sunday's referendum, which lasted three days, was unilaterally organised by the Ngok Dinka, a rival tribe closely connected to South Sudan.
Observers said the referedum, endorsed neither by Sudan nor south Sudan, was a "threat to peace".
The fate of Abyei is one of the most important and sensitive issues left unresolved since South Sudan became an independent state in 2011, ending two decades of civil war in Sudan.
"The laborious process of counting the number of ballots cast has begun," Tim Flatman, an independent observer in Abyei, said, calling the counting a "slow but very transparent process".
A majority of 65,000 registered voters are believed to have cast their ballots in the exercise, which closed on Tuesday evening.
Ballot papers were marked with two symbols to choose from: a pair of clasped hands symbolising a vote to be part of Sudan, and a single hand if people want to join South Sudan.
Results were expected later on Wednesday or Thursday.
Nkosazana Dlamini-Zuma, the AU chief, said the vote was illegal and its organisers were risking starting a return to war between civil war foes in Juba and Khartoum.
"They pose a threat to peace in the Abyei area, and have the potential to trigger an unprecedented escalation on the ground ... with far-reaching consequences for the region as a whole," she said in a statement on Monday.
Sudan and South Sudan have previously clashed over Abyei, which is patrolled by some 4,000 Ethiopian-led UN peacekeepers.
Although organisers insisted the referendum was open to all Abeyi residents, only the Ngok Dinka voted.
The Misseriya angrily said they will not recognise the result.
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 two-decade long civil war.
That referendum was repeatedly stalled, and Sudanese troops stormed Abyei, the size of Lebanon, in May 2011 forcing over 100,000 to flee southwards, leaving a year later after international pressure.