"Preparations for the Abyei referendum are even further behind schedule, and the continued lack of progress is exacerbating an already tense and volatile situation on the ground," Ban said in a report to the Security Council.

He told there is an urgent need to form the Abyei referendum commission to address and tackle the concerns of the local people. He urged to ensure that the vote is held on time.

Ban said in his speech that the southern Sudan commission must now work "extremely quickly" if it is to get a vote ready on time.

Sudan ambassador has warned that war could re-erupt if the two sides did not reach an agreement about Abyei region before the referendum.

"It is evident that any attempt to conduct the plebiscite before achieving an acceptable settlement between the two parties will mean only a return to war," Sudan's ambassador to the UN, Daffa-Alla Elhag Ali Osman Osman told a Security Council debate on Sudan.

The ambassador said his government wanted negotiations on the referendums to lead to peace "not to war, which we have never willingly accepted."

The vote on Abyei region is scheduled to take place at the same time as the referendum on southern independence. Abyei residents would decide whether to remain in the north or join the south.

It was part of the 2005 deal that had ended Africa’s longest civil war that killed about two million people and displaced another four million.

Southern Officials said it is unacceptable to postpone the vote from the January deadline. US backed talks on the future of South Sudan's oil-rich Abyei region in Addis Ababa failed its first phase but the two sides are due to meet in Addis Ababa on Wednesday.

Khartoum and Juba are accusing each other of a military presence on their frontier but the UN mission in Sudan known as UNMIUS had already increased monitoring along the frontier and reinforcing their presence at border.

The mission chief said that even a reinforce UN force in Sudan could not stop conflicts between the north and the south if tensions over vote boil over.

UN peacekeeping chief Alain LeRoy told the Security Council that any redeployment would weaken the peacekeeping mission in the rest of Sudan but added: "Any increase in the number of troops would not enable UNMIS to prevent or contain a clash on the frontier."

"Our best available tool against a return to war remains our commitment in favor of a political agreement... of the parties on the key pending issues," he said.

The north and south peace deal in 2005 had ended Africa’s longest civil war that killed about two million people and displaced another four million.