GENEVA — One day after the South Sudanese opposition rejected a peace deal in the country’s yearslong civil war, United Nations officials on Tuesday outlined a campaign of brutal killings and rapes carried out by government forces and their allies this spring.In the offensive, which targeted opposition-controlled villages in the north of the country in April and May, government troops and allied militias gunned down fleeing civilians, strung up villagers from trees and gang-raped women and girls — some of them fatally — investigators said in a 17-page report.At least 232 civilians were killed, and the forces gang-raped at least 120 women and girls, including children as young as 4, according to the report, which was produced by the United Nations mission in South Sudan and the human rights office in Geneva. At least 132 women and girls were abducted, forced to carry loot to the soldiers’ base and kept as sex slaves or porters, according to the report.The attacks underscored the horrific toll on civilians in the civil war that erupted in 2013 between South Sudan’s Dinka ethnic majority, led by President Salva Kiir, and the ethnic Nuer aligned with his former vice president, Riek Machar, just two years after the country gained independence from Sudan.Tens of thousands have died in the nearly five-year conflict, around two and a half million have fled to neighboring countries, and millions more have been left fending off starvation in South Sudan’s devastated economy.After a series of talks hosted by the leaders of neighboring countries in recent weeks, President Kiir unveiled an agreement on Sunday that would have reinstated Mr. Machar as vice president. But on Monday the opposition said the deal was unacceptable.Previous peace deals and cease-fires had also fallen apart.As the latest agreement crumbled, the United Nations human rights chief, Zeid Ra’ad al-Hussein, insisted that “the perpetrators of these revolting acts” in the springtime offensive — including those issuing the orders — “must not be allowed to get away with it.”Investigators described how groups of 100 or 200 soldiers and youth militia fighters attacked villages before dawn in operations between April 16 and May 24. Witnesses said that many who were unable to flee — including sick, older and disabled people, and children — were burned alive in their homes or had their throats slit, according to the investigators. Women who resisted rape were shot, and villagers who did not comply with demands to hand over money were dragged away and hanged from trees, the United Nations said.The death toll was probably higher than the number reported, investigators said. It was not known how many people were killed in shelling of the swamps and river islands where villagers sought refuge, but at least 10 children drowned as they fled into the swamps.“How can I forget the sight of an old man whose throat was slit before being set on fire?” a 14-year-old girl from the area told United Nations investigators. “How can I forget the smell of those decomposed bodies of old men and children pecked and eaten by the birds, those women that were hanged and died up in the tree?”The attackers’ ruthlessness suggested a “scorched-earth approach” intended to ensure that civilians never returned to the opposition-controlled areas, Ravina Shamdasani, a United Nations human rights spokeswoman in Geneva, told reporters.The United Nations also identified three commanders who may bear the greatest responsibility for the reported atrocities, Ms. Shamdasani said.The African Union and South Sudan’s government have drawn up plans for a court with international judges to prosecute the worst crimes of the conflict, but have yet to reach an agreement to put it into operation.Mr. al-Hussein, the United Nations human rights chief, urged them to move quickly to establish it. “The government of South Sudan and the international community have the obligation to ensure justice,” he said.