The violence among rival armed groups reignited in mid-August in Upper Nile State, in the north of the country, dpa news agency quoted a statement issued on Wednesday by the Mission as saying.
UNMISS said 200 people had sought safety in a UN refugee camp in Makalal, the provincial capital of Upper Nile State.
Many children are reported to be missing or separated from their families.
Their plight has been compounded by rainy season flooding, as it hinders the transportation of vital humanitarian aid needed to provide for the refugees.
South Sudan, with a population of about 11 million, has been led by a fragile transitional government under President Salva Kiir and former rebel leader Riek Machar since early 2020.
In 2013, civil war broke out in the oil-rich country, which has been wracked by decades of conflict, killing thousands and causing millions to flee.
Despite a peace agreement five years ago, various armed groups continue to fight for regional supremacy.
UNMISS said that it “urges feuding parties to immediately stop the violence” and “calls on national and local authorities to take immediate measures to protect civilians and prevent further escalation of the clashes”.
