Amnesty International's report, based on research following a government offensive on Leer and Mayendit counties in the northern Unity State between April and June, catalogued the testimonies of around 100 civilians who escaped the attacks.
"The offensive was characterised by staggering brutality, with civilians deliberately shot dead, burnt alive, hanged in trees and run over with armoured vehicles," Amnesty said.
The group also documented "systematic sexual violence", rape and gang-rape as well as abductions of women and girls, and the deliberate killing of young boys and male infants.
The killings echo the type of brutality meted out to civilians that has characterised South Sudan's war since the start.
Amnesty said the latest offensive began in April and continued until early July, "a week after the latest ceasefire was brokered on 27 June".
That ceasefire paved the way for the signing last week of another peace agreement between President Salva Kiir and rebel leader Riek Machar aimed at ending the vicious five-year-old civil war that has killed tens of thousands of people, pushed millions to the brink of starvation and scattered refugees across East Africa.
'End impunity'
The battle for power between Kiir, a member of the Dinka tribe, and Machar, a Nuer, meant the conflict quickly took on an ethnic character with civilians targeted by both sides for massacre and widespread rape.
UN rights experts have warned of "ethnic cleansing" and the threat of genocide.
Amnesty blamed a failure to prosecute perpetrators for the continuing violence.
"The only way to break this vicious cycle of violence is to end the impunity enjoyed by South Sudanese fighters on all sides," said Joanne Mariner, Amnesty's senior crisis advisor.
In a rare example of justice, 10 soldiers were found guilty earlier this month of an attack on a hotel in the capital Juba in which five foreign aid workers were gang-raped and a South Sudanese journalist killed.
But commanders and their political masters are not held to account.
A so-called "hybrid court" to try war crimes and crimes against humanity, proposed by the African Union as part of a failed 2015 peace agreement, has not been set up.
South Sudan, the world's youngest nation, broke away from Sudan in 2011 after a long and bloody independence struggle, but just two years later the new war began triggering one of the world's worst humanitarian crises.
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