The first group of ethnic South Sudanese among up to 15,000 camped in crowded conditions in Sudan began their journey home on Saturday, the International Organization for Migration said. About 400 people, mostly adults, were traveling to Khartoum by bus from the way-station of Kosti, about 300 kilometers (190 miles) south of the capital, ahead of a major airlift early on Sunday, IOM Sudan chief Jill Helke said. \"They\'re moving in a convoy,\" escorted by local government agencies, Helke told AFP. After staying overnight in a Khartoum-area government-run transit center, they are to board three aircraft shortly before sunrise on Sunday, at 0300 GMT, for the South Sudanese capital Juba. The IOM estimates that 12,000-15,000 South Sudanese are in the Kosti way-station. Many have been living in makeshift shelters or barn-like buildings, waiting 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 whom the South Sudanese embassy estimates remain in the north after an April 8 deadline for them to either formalize their status or leave Sudan. Hundreds of thousands of others have already gone to the South, 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. The aircraft are to return to Khartoum Sunday afternoon to collect another load of South Sudanese, a schedule which is to continue daily, Helke said. At that rate, the airlift could take about two weeks, but she said it is not yet clear how many Kosti residents 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, which the United Nations called 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. Since last year the IOM has helped return more than 23,000 Southerners, mostly by river barge.