The last flight in an airlift of about 12,000 ethnic South Sudanese, ordered to leave by local authorities in Sudan, is to take off this week, the International Organisation for Migration said on June 4.

The IOM began more than three weeks ago the charter flights to transport Southerners who had been staying at the Kosti way-station 300 kilometres from Khartoum. Kosti became home to the biggest single concentration of South Sudanese needing transport South, with many living in makeshift shelters or barn-like buildings for up to a year, and dependent on foreign aid.

The governor of the Kosti area declared the migrants a threat to security and the environment and ordered them out by May 5, sparking concern from the United Nations and the IOM which has already helped thousands of South Sudanese head to South Sudan, which became independent last July. Officials extended the deadline to May 20 but then told the IOM to disregard the time-limit after plans for the airlift were devised. "This is very exceptional," said Jill Helke, the IOM's chief of mission in Sudan. "We only stepped in to do this by air because of the urgency" of moving people told to leave who had been already waiting for months. As of Monday morning, 11,020 people had flown out on 73 flights. Two more planes were to leave for the South Sudanese capital Juba that day followed by another Tuesday and the last one on Wednesday, Helke said, adding the airlift had gone "remarkably well" considering it is the season for dust storms.

About 900 people in Kosti are still registered for the airlift but Helke said they are not all expected to travel. An unknown number managed to arrange truck transport to South Sudan by themselves.

The South Sudanese in Kosti were among about 350,000 ethnic Southerners who the South Sudanese embassy estimated remained in the north after an April 8 deadline to either formalise their status in the north or leave the country.

Many have spent their entire lives in the north or came to Sudan when they were children, as millions fled a 22-year civil war. The war ended in a 2005 peace deal which led to South Sudan's independence.