An airlift of up to 15,000 ethnic South Sudanese aims to start from the Sudanese capital early Monday after a series of delays, the International Organisation for Migration said. \"We hope that we will have one plane leaving either 9:00 or 9:30 tomorrow morning (0600 or 0630 GMT),\" Jill Helke, who heads the IOM in Sudan, told AFP on Sunday. The first group of about 400 South Sudanese, mostly adults, travelled to Khartoum by bus on Saturday from the way-station of Kosti, 300 kilometres (190 miles) south of Khartoum. They were expecting to take flights to the South\'s capital Juba early Sunday but a variety of factors have caused a delay in the airlift, Helke said, citing visa problems and insufficient rest time for the air crews. The IOM estimates 12,000-15,000 South Sudanese are in the Kosti way-station. 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. 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 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 South Sudanese who have already been transported from Kosti are waiting for their flights at a government-run transit centre in the Khartoum area. Plans have called for six daily flights during the airlift, but Helke said they now aim to start with a 145-seater plane followed by a possible second flight later on Monday.