UN peacekeepers evacuated 220 staff and aid workers from northeast South Sudan on Wednesday after militias killed at least six relief workers, a UN spokesman said.
The troops from the UN mission in South Sudan were expected to continue evacuations tomorrow from Bunj, in Upper Nile State, where a local militia -- the Mabanese Defense Forces - was blamed for the killings.
"The peacekeepears are in the process of collecting national staff members of humanitarian aid organisations deemed to be at risk and who need to be evacuated as soon as possible," said deputy spokesman Farhan Haq.
The violence in Upper Nile State came as South Sudan is facing what the UN has dubbed the worst food crisis in the world and peace talks between rival factions have yet to yield concrete results in Ethiopia.
At least five South Sudanese aid workers were killed yesterday by the militia blamed for murdering an aid worker in the same area the day before, according to the UN.
The Mabanese Defense Forces appear to be targeting civilians of ethnic Nuer origin, apparently in revenge for losses they suffered in clashes with defecting Nuer soldiers.
In Washington, US Secretary of State John Kerry condemned the violence and called for progress in peace talks.
"The people of South Sudan are paying a horrific price for the failure of their leaders to resolve this senseless conflict," he said.
Fighting erupted in December, sparked by a power struggle between President Salva Kiir and his sacked deputy Riek Machar.
A UN Security Council visit to South Sudan next week is expected to underscore the message to the two leaders that they must stop the fighting or face consequences.
Diplomats have raised the possibility of sanctions targeting the two leaders, but such a measure would be decided after the visit to Juba.
Thousands of people have been killed and over 1.5 million have fled the fighting between government troops, mutinous soldiers and ragtag militia forces divided by tribe.