The United Nations Mission in South Sudan (UNMISS) said civilians had been “deliberately and brutally targeted” in Central Equatoria since the agreement was inked in September.
At least 104 people had been killed in attacks on villages in the southern region, it said, Presstv Reported.
A roughly similar number of women and girls were raped or suffered other sexual violence between September and April, it said in its latest human rights report.
Many were taken captive by armed groups to serve as “wives,” it said.
The surge in violence has forced more than 56 000 civilians to flee their homes, becoming displaced in South Sudan itself, while another 20 000 have crossed the border into Uganda and the Democratic Republic of Congo.
South Sudan descended into war in 2013 when President Salva Kiir accused his former deputy and fellow former rebel leader Riek Machar of plotting a coup.
The conflict has been marked by ethnic violence and brutal atrocities, and left about 380,000 dead while some four million have fled their homes.
UNMISS said overall there had been a “significant decrease” in violence across the country since Kiir and Machar signed the deal.
“However, Central Equatoria has been an exception to this trend, particularly in areas surrounding Yei, where attacks against civilians have continued,” the report said.
The report identified government forces, fighters allied to Machar and rebel groups who did not sign the peace agreement, as responsible for atrocities in their quest to take territory in Central Equatoria.
In the first phase of fighting, that coincided with the signing of the peace agreement, at least 61 civilians were killed in deliberate attacks or caught in indiscriminate crossfire.
“At least 150 civilians were also held in captivity by these groups, including women and girls taken as ‘wives’ by commanders or raped and beaten by multiple fighters,” the report said.
The second outbreak of violence began in January, when government forces punished those believed to be rebel collaborators with “sexual violence as well as looting and destroying homes, churches, schools and health centres”.
Under the peace deal, Kiir agreed to set up a unity government with longtime rival Machar, who is to return from exile.
But this new government, initially scheduled to take office on May 12, was postponed for six months.