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.