International medical organisation Medecins Sans Frontiers has withdrawn its international staff from several clinics in southern Sudan.
Many people have returned home since the end of the war
The agency says attacks by militia have made large areas in the provinces of Upper Nile and Jonglei too dangerous to operate in.
The violence began in April, when an attack was carried out in the village of Ulang, where MSF operated a clinic.
Last year's peace deal was supposed to end 21 years of war in southern Sudan.
"We are concerned about the growing number of violent incidents," says MSF co-ordinator Cristoph Hippchen.
"This means humanitarian assistance to the people of Upper Nile and Jonglei, already far below what is needed, will be even less now."
MSF's 50 international staff and 400 Sudanese staff were providing medical assistance in an area the size of France, but now half of these facilities have been closed.
In the Ulang raid, staff fled and 31 hospital patients were killed and dozens injured.
Last year's peace agreement between the government in Khartoum and the main southern rebel group, the SPLA was supposed to end the violence, requiring all militia groups to integrate into the SPLA, or be disbanded.
But a number of organisations are refusing to comply, and in some areas the SPLA is attempting to disarm them by force.