Reports say children in South Sudan have been forced to watch their mothers being raped and killed.
A report by UN human rights investigators says that 40 officials may be individually responsible for war crimes and crimes against humanity.
It says civilians have been tortured and mutilated, and villages destroyed on an industrial scale.
Conflict between government factions has continued in South Sudan despite a peace deal signed in 2015.
The UN investigators collect evidence for use in future war crimes trials, and the report will be presented to the UN human Rights Council in Geneva.
Of the 40 senior officials identified as potentially responsible for atrocities, five are colonels.
But the court has still not been set up because South Sudan’s parliament has not yet approved it.