The clashes between forces loyal to President Salva Kiir and his rival, Vice President Riek Machar, in oil-rich Unity State affected at least 28 villages across three counties, with 173 people killed and 37 women and children kidnapped.
"Many of the abductees were subjected to sexual violence, including girls as young as eight-years-old and a nine-year-old girl who was gang-raped to death," the UN Mission in South Sudan (UNMISS) and the Office of the UN High Commissioner for Human Rights (OHCHR) said.
Both sides committed severe abuses, the report said, adding that pro-government forces and militias loyal to Kiir appeared to be "the main perpetrators of the human rights violations".
The violence caused 44,000 people to flee their homes across 26 villages, with a total of 131 cases of rape and gang-rape documented.
South Sudan has been wracked by instability since independence in 2011 and is still struggling to draw a line under a civil war between pro-Kiir and pro-Machar fighters that claimed the lives of almost 400,000 people.
The joint report covered the period between 11 February and 31 May 2022, with researchers travelling to the pro-Machar strongholds of Koch, Leer, and Mayendit as well as surrounding areas to document the aftermath of the violence.
It said that there were "reasonable grounds to believe that these attacks were consistently premeditated and carried out with a degree of organization mainly by the joint Government forces and allied militias/groups operating in these areas."
In a press statement accompanying the report's release, Nicholas Haysom, the UN envoy to the country, said that "human rights violations were committed with impunity."
"The government is duty-bound under international law to protect civilians, investigate allegations of human rights violations, and hold suspected perpetrators accountable," he added.
The UN has regularly criticized South Sudan's leadership for its role in stoking violence, cracking down on political freedoms and plundering public coffers.
It has also accused the government of rights violations amounting to war crimes over deadly attacks in the southwest last year.
Since the five-year civil war ended in 2018, the country's lumbering peace process has run into multiple delays, with violence regularly breaking out between Kiir and Machar's forces.
In July, the United States pulled out of two peace process monitoring organizations in South Sudan due to the government's failure to meet reform milestones, citing a "lack of sustained progress".