South Sudanese pro-government forces killed at least 114 civilians in and around Yei town between July 2016 and January 2017, as well as committing uncounted rapes, looting and torture, the UN human rights office says.
"Attacks were committed with an alarming degree of brutality and, like elsewhere in the country, appeared to have an ethnic dimension," a report on the UN investigation said.
"These cases included attacks on funerals and indiscriminate shelling of civilians, cases of sexual violence perpetrated against women and girls, including those fleeing fighting, often committed in front of the victims' families."
Fighting flared when the Sudan People's Liberation Army (SPLA), loyal to President Salva Kiir, pursued his rival and former deputy Riek Machar and a small band of followers as they fled from the capital Juba, southwest through Yei and into neighbouring Congo.
The pursuit of Machar ushered in a particularly violent period in South Sudan's Equatorias region, with multiple localised conflicts, particularly in Yei, the report said.
South Sudan army spokesperson Colonrl Santo Domic Chol told Reuters the report was "baseless".