|A large number of southerners live in Khartoum, the northern capital [Reuters]
Thousands of southerners have returned to south Sudan from the north ahead of the independence referendum that could see the south split from the north to form a separate state.
International envoys including Jimmy Carter, the former US president, Thabo Mbeki, the former South African president, and Kofi Annan, the former United Nations secretary-general, also arrived in the region on Friday to monitor the vote.
The United Nation's refugee agency said on Friday that an average of 2,000 people were crossing back into the south each day, with the total figure in recent weeks at more than 120,000.
"The number of southerners who are leaving the North ahead of this week's landmark Sudan referendum to return to their ancestral homes in the South has doubled since mid-December and now stands at 120,000," the office of the UN High Commissioner for Refugees (UNHCR) said in a statement on Friday.
Some reports say that hundreds more southerners have been stranded south of Khartoum, the northern capital, waiting for transport back to the south.
The UNHCR said that it expected many more people would return in the months following the January 9-15 referendum.
"Many of the returnees who have lived in the north for years say they have left for fear of the unknown and the opportunity to start afresh in their native South," the UN refugee agency said.
The Carter Center, founded by the former US president, is deploying at least 75 observers to the monitor the referendum, while China, which has large investments in Sudan's oil sector, is also sending observers, as is the European Union.
Asked by Al Jazeera what his message was to people in the north and the south, Carter said: "Just accept the decision of the southerners in the referendum peacefully, no matter whether it is to stay part of Sudan or to form a new nation.
"Then after that, devote full time to the complete implementation of the Comprehensive Peace Agreement provisions dealing with Abyei, dealing with the borders and and divisions of wealth, in particular oil."
The 2005 Comprehensive Peace Agreement (CPA), which brought an end to civil war between the north and south, stipulates a quorum of 60 per cent of the 3.8 million registered voters, meaning that while there has been little apparent opposition to southern independence, voter turnout will be crucial.
Mbeki, who is chairperson of the African Union (AU) High Level Implementation Panel for Sudan, was upbeat about the referendum and other issues arising from the 2005 peace deal, saying that he expected them to be resolved before the CPA expires on July 9.
Speaking in Juba on Friday Mbeki said that under his AU initiative the Doha peace process on Darfur would be broadened and that other unresolved issues like border delineation, citizenship and the division of Sudan's national debt will taken up in a "framework document" of post-referendum issues.
Also in Sudan for the referendum is George Clooney, the Holywood star. He arrived in Juba, the southern capital on Thursday and was due to travel to the disputed region of Abyei on Friday.
The oil-rich Abyei region had been due to hold a simultaneous vote on its future on Sunday, but that has now been indefinitely postponed.
The delay has prompted fears of clashes between the district's settled pro-southern Ngok Dinka population and Misseriya Arabs from the north who use its waters for seasonal pasture.
Abyei is the sticking point in what is otherwise being viewed as a successful build up to the landmark vote.
On Friday, the election commission organising the referendum said that preparations were complete.
"Preparations are absolutely complete - the ballots arrived at all of the centres all over the south," George Mater Benjamin, the commission spokesman, said.
"We have co-ordination with the United Nations. They are giving us small planes to distribute the ballots."
Earlier, in a rare act of praise for Omar al-Bashir, the Sundanese president, who is wanted by the International Criminal Court, the UN Security Council said its members "appreciate" his stance on the referendum.
Al-Bashir has said he will respect and support the south's wishes if it chooses independence.
David Gressly, the southern head of the United Nations Mission to Sudan, expressed optimism that calm would be maintained for the referendum.
"For the last several weeks - several months probably - we have seen the lowest level of insecurity in southern Sudan since the Comprehensive Peace Agreement has been signed," he said.
"It has been a very conducive environment for the registration, and we expect the same to continue for the polling period."
But aid agency Oxfam took issue with the peacekeeping mission's focus, saying it should have devoted fewer resources to the referendum and more to the protection of civilians from persistent low-intensity violence that has claimed nearly 1,000 lives over the past year.
"The United Nations peacekeeping mission in Sudan has so far failed to prioritise the protection of local people," it said.