The UN human rights office said its monitors had in recent months documented "what appear to be deliberate, ruthless and brutally violent attacks on civilians.
The UN human rights office said its monitors had in recent months documented "what appear to be deliberate, ruthless and brutally violent attacks on civilians, particularly against women and children".
The agency said at least 234 civilians were killed and many more injured in attacks between April 16 and May 24 by government and aligned forces and armed youth in villages in opposition-controlled areas in Mayendit and Leer in Unity State.
The agency said a "scorched earth" approach, and warned that some of the gross violations documented in its report might amount to "war crimes".
"Civilians were targeted, with the elderly, people with disabilities and very young children killed in horrific acts of violence," it said in a statement, pointing to people being hung from trees and others burned alive in their homes.
The report also documented the use of rape and other sexual violence as a weapon of war, highlighting that at least 120 women and girls had been raped or gang-raped, including children as young as four.
At least 132 women and girls had been abducted, it found.
A 20-year-old woman from Leer County who had fled into the bush with her three-day-old baby told investigators that she was still bleeding from labour when a soldier raped her.
More than 5,000 people had sought sanctuary at UN protected sites in Leer and Bentiu, and another 18,000 had sought refuge in Mayendit town, the UN said.
Another 8,000 were estimated to be hiding in the bushes and swamps, it added.
UN High Commissioner for Human Rights Zeid Ra'ad Al Hussein called on the South Sudanese government to halt attacks on civilians and to ensure those behind the violence are brought to justice.
"The perpetrators of these revolting acts against defenceless civilians, including those bearing command responsibility, must not be allowed to get away with it," he said.
The UN rights chief said "there must be consequences for the men who reportedly gang-raped a six-year-old child, who slit the throats of elderly villagers (and) hanged women for resisting looting".
He urged South Sudan and the African Union to move quickly to establish a hybrid court to try those responsible for serious crimes.
Two years after separating from Sudan in 2011, oil-rich South Sudan plunged into a war that has since killed tens of thousands of people and displaced four million, after President Salva Kiir accused his then-deputy Riek Machar of plotting a coup.
The warring leaders agreed in Uganda Saturday to a power-sharing deal intended to see Machar return to his position as vice president, although details are still being discussed.
A similar power-sharing deal that returned Machar to the vice presidency was signed in 2015 but fell apart a year later in a deadly battle that saw Machar flee into exile.