"Grave human rights violations and abuses, including hundreds of killings, were committed against civilians during fighting in Tambura County, Western Equatoria State," according to a joint report by the United Nations Mission in South Sudan (UNMISS) and the UN Human Rights Office.
It said that between June and September last year, at least 440 civilians were killed, 18 injured and 74 abducted during clashes between rival groups.
At least 64 civilians were also subjected to conflict-related sexual violence, among them a 13-year-old girl who was gang-raped to death, the report said, while at least 56 people were also reported missing during the June-September period.
In addition, some 80,000 were forced to flee their homes to escape fighting, it said.
"Looting and destruction of property, child conscription, attacks on personnel and facilities, hate speech and incitement to violence were among the other human rights violations the investigation uncovered."
The report blamed members of the armed forces loyal to President Salva Kiir and rivals in the Sudan People's Liberation Movement/Army in Opposition (SPLM/A-IO) of Deputy President Riek Machar, and "their respective affiliated militias".
"We call on all parties to the conflict to hold to account all individuals implicated in the killings, rape and abductions, among other grave human rights violations," UN High Commissioner for Human Rights Michelle Bachelet said in a statement.