An official from the south's main party told Reuters it would be "unacceptable" to postpone the politically sensitive vote from the Jan. 9 deadline set in a 2005 peace deal.

He said Abyei residents might be forced to hold their own referendum without northern approval, raising the prospect of a diplomatic rift or even conflict.

Both north and south Sudan claim the oil-producing region and fought over it during a decades-long civil war that left an estimated 2 million people dead.

Abyei residents were promised a referendum on whether to join north or south Sudan as part of the 2005 deal that ended that conflict. Relations between the two former foes have remained deeply troubled and northern and southern forces have already clashed in the area since the accord.

Northern officials told journalists Thursday they had failed to resolve a bitter dispute with southerners over who should take part in the referendum and they had now run out of time to hold the vote as scheduled.

"It is very clear that right now it is not possible to have the Abyei referendum on 9 Jan., 2011. We all agree that this is no longer practical," Didiri Mohammad Ahmad, a senior member of Sudan's ruling National Congress Party (NCP), told journalists.

"We agreed that in the next talks we will try to look for other alternatives."

One of those alternatives, he added, was for the sides to meet and reach "a conclusion on the final status of the Abyei area" themselves.

Sudan's minister of international cooperation, Jalal Yousif Al-Dagir, told the news conference the government would be open to a proposal to delay the referendum by four months or more.

"The referendum is not completely out of the window but it is apparent now that other solutions may be the real alternative," he said.

Abyei's administrator said its residents would not accept a delay and may hold their own vote without the government.

"A delayed vote is unacceptable. The people of Abyei are still holding out for the referendum to be held on Jan. 9. If the government does not give them that option we can have a self-run referendum," said Deng Arop Kuol, a member of south Sudan's dominant Sudan People's Liberation Movement (SPLM).

Kuol declined to comment on whether he would accept any alternative to the Abyei vote, but said he and the SPLM would return to negotiations with the north in Addis Ababa, scheduled for the end of the month.

The last round of negotiations on Abyei between the parties, attended by US Ambassador to Sudan Scott Gration, ended in Addis Ababa Tuesday without agreement.