South Sudanese government troops held women as sex slaves while attacking scores of civilians just outside UN bases, according to a UN report.
The report said hundreds of people, including pregnant women and under-18s, were raped in Unity state by soldiers from either side.
Tens of thousands have been killed in the conflict, which started more than two years ago, as government forces battle rebels.
The UN said sexual violence was centred in oil-rich Unity, and government soldiers were responsible for most incidents.
At least 35 women were raped by government soldiers in two months of 2015 outside their base in the Unity capital, Bentiu.
The government denied the reports of rape and other crimes, according to an official response published along with the UN report.