The first group of ethnic South Sudanese among up to 15,000 camped in crowded conditions in Sudan has begun their journey home.
Roughly 400 people, mostly adults, travelled to Khartoum by bus on Saturday from a town 300 kilometres south of the capital ahead of a major airlift that had been planned for early Sunday, said Jill Helke, country director for the International Organisation for Migration.
But the airlift has been delayed, she said.
"The early morning flights are not going to go for a number of reasons," including because the air crews were delayed in arriving and have not had the required rest, she told the AFP news agency.
One flight might take off at about midday on Sunday for the South Sudanese capital Juba, Helke said.
The South Sudanese were to spend the night in a government-run transit centre in the Khartoum area.
The IOM estimates that 12,000 to 15,000 South Sudanese are in Kosti, the southern Sudan town. Many have been living in makeshift shelters or barn-like buildings, waiting several months for their transport home.
The governor of the area declared the migrants a security threat and initially gave them a May 5 deadline to leave, sparking concern from the United Nations and the IOM, which has already helped thousands of South Sudanese to head home.
Sudanese officials extended the deadline to May 20 but then told the IOM to disregard the time limit after plans for the airlift were devised.
If the deadline is enforced, "this becomes a deportation and we will not have any part in it," Helke said.
The South Sudanese in Kosti are among about 350,000 ethnic southerners who the South Sudanese embassy estimates remain in the north after an April 8 deadline for them to either formalise their status or leave Sudan.
Hundreds of thousands of others have already gone to South Sudan, which separated last July under a peace deal at the end of a 22-year civil war, which killed two million people and drove many more to the north.
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Plans called for six daily flights, a rate which would require about two weeks to move all the southerners from Kosti, but Helke said it is not yet clear how many of them will join the flights.
"We've been screening about 900 people a day," she said. "There hasn't been an overwhelming rush."
Some have managed to arrange truck transport to South Sudan by themselves. Helke said interest in the IOM flights could pick up after South Sudanese government vehicles begin moving the passengers' luggage on Saturday.
Luggage is a big concern for the returnees, who want to bring as much as possible to the poverty-stricken South.
The airlift will take place under tight security because Sudanese authorities are worried that South Sudanese in Khartoum might also try to join the flights.
Kosti way-station on the White Nile River was designed for about 2,000 people but became home to the biggest single concentration of South Sudanese awaiting transport. Other groups have been living in rough conditions in Khartoum.
The IOM said all the southerners in Kosti were dependent on assistance from the international community for food, water, health care and other essential services and most did not have their own means to arrange transportation.
Helke said the IOM, which is dependent on donor funding, has money to pay for about half the airlift and is urging donors to provide the rest.
The IOM had plans for moving thousands of people from Kosti by barge but Sudan's military expressed security concerns.
Since late March, Sudan and South Sudan have been fighting along their border, in what the United Nations termed a serious threat to international peace and security.
A UN Security Council resolution on May 2 ordered both sides to cease hostilities and to resume by next Wednesday negotiations on unresolved issues including the status of each country's nationals in the other country.
The IOM has since last year helped return more than 23,000 Southerners, mostly by river barge.