South Sudanese refugees begin journey to Juba
The first group of South Sudanese refugees stranded for months in a camp in Sudan is on its way to the south.
About 400 people - out of a total of up to 15,000 - are being taken by bus to Khartoum from where they will be flown to the South Sudanese capital Juba.
The camp residents were last month declared a security threat by the authorities, who gave them a deadline to leave Sudan. This was later dropped.
They lost rights to Sudan nationality when the south seceded last year.
The airlift comes a day after UN human rights chief Navi Pillay condemned Sudan's bombing of South Sudan, carried out despite a UN resolution demanding an end to hostilities.
South Sudan seceded last July as part of a deal to end years of civil war.
But disputes stemming from the secession, especially over oil, led to clashes last month and fears of a return to all-out war.
On Wednesday, South Sudan accused its neighbour of continued bombing raids. Khartoum said it had the right to respond to acts of aggression.'Lack of choice'
The group of South Sudanese is being taken to Khartoum from Kosti in White Nile State, about 300km (190 miles) south of the capital.
They will be flown out of Khartoum early on Sunday morning after spending the night at a government transit centre, International Organisation for Migration (IOM) head Jill Helke told AFP news agency.
The IOM says the 15,000 have been stuck in Kosti for anything up to a year, after boat traffic south was cut because of cross-border violence.
They have been living in makeshift shelters and are entirely dependent on aid agencies as they wait for transport.
"Part of the problem has been that in the period since independence, although the people were supposed to be given a choice to leave or to become legalised, there have not been the arrangements available for people to legalise their stay in Sudan," Ms Helke told the BBC.
"And so for many the lack of choice has made them decide that going to South Sudan is their only option."
Correspondents say many of the group have never been to South Sudan, so settling there is likely to be a challenge.
Many have relatives in the south and the IOM and other humanitarian agencies will be providing some help, the aid agency said.
Hundreds of thousands of South Sudanese left Sudan after the south seceded, but about 350,000 remain.
Both Sudan and the South are reliant on their oil revenues, which account for 98% of South Sudan's budget. But the two countries cannot agree how to divide the oil wealth of the former united state. Some 75% of the oil lies in the South but all the pipelines run north. It is feared that disputes over oil could lead the two neighbours to return to war.
Although they were united for many years, the two Sudans were always very different. The great divide is visible even from space, as this Nasa satellite image shows. The northern states are a blanket of desert, broken only by the fertile Nile corridor. South Sudan is covered by green swathes of grassland, swamps and tropical forest.
Sudan's arid north is mainly home to Arabic-speaking Muslims. But in South Sudan there is no dominant culture. The Dinkas and the Nuers are the largest of more than 200 ethnic groups, each with its own languages and traditional beliefs, alongside Christianity and Islam.
The health inequalities in Sudan are illustrated by infant mortality rates. In South Sudan, one in 10 children die before their first birthday. Whereas in the more developed northern states, such as Gezira and White Nile, half of those children would be expected to survive.
The gulf in water resources between north and south is stark. In Khartoum, River Nile, and Gezira states, two-thirds of people have access to piped drinking water and pit latrines. In the south, boreholes and unprotected wells are the main drinking sources. More than 80% of southerners have no toilet facilities whatsoever.
Throughout the two Sudans, access to primary school education is strongly linked to household earnings. In the poorest parts of the south, less than 1% of children finish primary school. Whereas in the wealthier north, up to 50% of children complete primary level education.
Conflict and poverty are the main causes of food insecurity in both countries. In Sudan, many of the residents of war-affected Darfur and the border states of Blue Nile and South Kordofan, depend on food aid. The UN said about 2.8m people in South Sudan would require food aid in 2013. The northern states tend to be wealthier, more urbanised and less reliant on agriculture.