South Sudan said Wednesday it was deeply concerned for the lives of more than 12,000 of its citizens trapped in neighbouring Sudan, ordered to leave by Saturday but without transport to travel southwards.
The Southerners in Sudan's White Nile river port of Kosti are part of a mass migration of people leaving Sudan after the South declared independence in July.
"These people can actually lose their lives, our concern is that these people might actually be killed, as the government of Sudan is not caring for them anymore," Southern Information Minister Barnaba Marial Benjamin told reporters.
Transport is limited, with many living in makeshift shelters in the town for months, but the governor of the Sudanese border state has said they threaten the security of the region and ordered them to be expelled by 5 May.
Sudan has blocked aid agencies from giving the returnees food, Benjamin claimed.
"They have actually removed all their food from the stores that was meant for this, they are preventing them to get drinking water," he said, adding that those that try to leave the camps risk arrest.
"These people have been there for nearly going to a year now with all their property, they are just waiting for barges to come," he said.
Over 375,000 ethnic Southerners— many of whom have spent decades in the north—have returned to the South since October 2010.
However, up to 500,000 remain in the north, despite the passing of an 8 April deadline for them to either formalise their status or leave Sudan.
The expulsion order for those in Kosti would be the first formal order for Southerners to leave, and follows weeks of deadly clashes between Khartoum and Juba in contested border areas.
Khartoum fears river barges could be used by the South to bring up weapons into border areas and has blocked their movement at the border.
The International Organisation for Migration (IOM) mission, which is supporting those who want to return to the South, said it was deeply concerned at the situation.
All international aid agencies have been ordered to leave Kosti by Saturday, but the international community does not have the logistical capacity to move the people in time, warning it would take months rather than days.
"To move by barge 12,000-15,000 people would take about 24 barges and over 60 luggage barges... without delays it would take four months," said IOM South Sudan spokeswoman Samantha Donkin.
Southerners could potentially walk across the frontier in a day, but would have to abandon all their belongings and would arrive in an area already struggling to cope with numbers of new arrivals.
The IOM has called on Khartoum to grant more time for Southerners "to move to South Sudan in safety and dignity."