Aid groups are mounting a "major emergency operation" in rural South Sudan after tribe-on-tribe violence sent tens of thousands fleeing and killed an unknown number of people, the United Nations said.
The UN's humanitarian co-ordinator in South Sudan, Lise Grande, said aid groups were responding to a call for help from South Sudan's central government after a column of 6,000 armed men from the Lou Nuer ethnic group marched into Pibor in Jonglei state to target the Murle community in late December and early January.
The UN says three villages were burned to the ground and aid groups evacuated 140 people who were wounded.
Neither the central government or the UN has been able to give a death toll. A government official in Jonglei has said thousands of people were killed, but that number has not been corroborated.
Aid groups estimated 60,000 people had been affected by the violence, the UN said. Tens of thousands fled their homes and urgent needs included high-nutritional food, clean water, healthcare and shelter.
"This emergency operation is going to be one of the most complex and expensive in South Sudan since the Comprehensive Peace Agreement was signed in 2005," Ms Grande said, adding that the areas most in need can be reached only by air.
"Delivering assistance by air is hugely expensive compared to delivery by road. Unfortunately, in the areas affected in Jonglei, we don't have a choice," she said.
The Red Cross said earlier this month it was trying to reconnect 150 young children with their missing parents after tens of thousands of residents of South Sudan ran into the bush while fleeing the impending attack. Many of those parents are feared to be dead.
Columns of fighters from the Lou Nuer ethnic group marched into Pibor to target the Murle community, two tribes that have traded violent attacks over the last several years that have killed thousands. Much of the communities' animosity stems from cattle raiding attacks.
South Sudan fought a decades-long civil war with northern neighbour Sudan, a war that culminated in a 2005 peace deal that saw the partitioning of Sudan and the birth of South Sudan last July.