The International Organization for Migration says it has begun airlifting 12,000 stranded South Sudanese from the Sudanese town of Kosti, south of Khartoum.
IOM spokesman Jean-Philippe Chauzy says his organization's chartered flights have been taking South Sudanese mainly to Juba, the capital of newly independent South Sudan.
He told reporters Tuesday in Geneva that many of them have been spent months living in tough conditions and are desperate to leave.
Millions of South Sudanese fled to northern Sudan during their 22-year war, and many of those were stranded in the north after South Sudan declared independence last July.
The IOM had warned that it lacked the logistics to meet a May 5 deadline set by Sudan's White Nile State to relocate up to 15,000 South Sudanese unable to leave Kosti.