Northern Sudanese officials have called for the postponement of a referendum on the future of Sudan's oil-producing Abyei region, but southern leaders have swiftly rejected the idea.

The vote on whether the region should join the north or the south is scheduled to take place on January 9, the same day as the south votes on independence.

Al-Dirdiri Mohammed Ahmed, chief negotiator for the ruling party, told reporters Thursday that northern officials have agreed that it is not possible to hold the vote on January 9.

But Abyei chief administrator Deng Arop Kuol, a member of the south's Sudan People's Liberation Movement, says the threatened delay is unacceptable and warned that residents could hold the vote on their own.

A U.S. State Department spokesman said Tuesday that the vote could still be held on time.

P.J. Crowley said both sides have committed to hold the referendum on January 9 and urged them to find a way to meet that commitment.

At the United Nations, U.S. ambassador Susan Rice said the south fears the north may be preparing for war ahead of the referendum.  She told the U.N. Security Council Thursday that the government of southern Sudan has asked the council during their visit to the country last week for a U.N. buffer zone along the north-south border.

Rice said most council members are skeptical about the feasibility that a U.N. force could line the entire border. However, she said there is serious discussion about the possibly of focusing troops at the most vulnerable areas of the border.

The referendums on Abyei and southern independence are key parts of the 2005 peace agreement that ended Sudan's north-south civil war.

Abyei is split between members of the northern-allied Missiriya tribe and the Ngok Dinka, who seem supportive of joining the south.

Current referendum laws give the Dinka voting rights, but a referendum commission has not yet decided what other groups are eligible to vote in that poll.

Some information for this report was provided by AP, AFP and Reuters.