Thousands of Sudanese are being forced to leave their homes and walk for days into an area already severely affected by food shortages.
Some 14,000 members of the Lou Nuer have been expelled from Upper Nile and are arriving in neighbouring Jonglei.
Their arrival is putting intense pressure on a population whose crops had all but failed.
Earlier this year, aid workers told the BBC that Akobo county was the "hungriest" place on earth.
Today in Akobo, food is in very short supply. The rains have arrived in Sudan, but the next harvest is still months away.
Nearly one in ten children in the region are so badly malnourished they are at severe risk of dying.
The charity Save the Children has already taken 600 children into its intensive feeding programme. And now thousands of people are arriving in the area, after being forced across the state border.
Local officials decided they had no rights where they were living, after the border was re-drawn.
Last Wednesday they were told to leave and now they have come, with the few possessions they can carry, to live with families to whom they are related.
Kate Foster of Save the Children says their arrival is stretching already meagre resources to breaking point.
"I think this is one of the most serious situations that we've seen in south Sudan this year," she said.
"The situation in the south Sudan was so bad already, particularly for children, and the lack of food that's available to them.
"It's really just going to be a very serious situation that will be compounded even further."
Akobo was already coping with more than 20,000 people who were displaced by fighting that took place last year.
A convoy of food being brought down the Nile was attacked in June. And ethnic clashes in August left 185 people dead, mainly women and children.
Now a situation of extreme malnutrition is being made even worse by the expulsion of people across the state border.