South Sudanese government soldiers killed more than 60 men and boys by keeping them in a shipping container until they suffocated and dumped their bodies in an open field, Amnesty International says.
The victims were arrested arbitrarily between October 20 and October 23 in Leer in the northern state of Unity, according to the report.
Africa's newest nation has seen widespread human rights violations since a power struggle between President Salva Kiir and his former deputy Riek Machar evolved into a military conflict in December 2013 that has killed tens of thousands and displaced more than 2 million people.
A peace agreement signed in August has stopped most of the fighting between the main warring parties, but militias also operate in the country.
The victims described in Amnesty's report had their hands tied and were forced to enter the container, which had no windows or ventilation.
Witnesses told the rights group they heard the detainees crying, screaming and banging on the walls of the container.
By the following morning, all but one of them had died.
"We could see the people inside and they were not alive ... The container was full of people. They had fallen over one another and on to the floor," one witness was quoted as saying.
Soldiers loaded the bodies into a truck and dumped them into open pits, according to the report, which was based on the testimonies of more than 40 people.
Amnesty's researchers found the remains of many broken skeletons still strewn across the ground.
"The arbitrary arrest, torture, and mass killing of these detainees is just one illustration of the South Sudanese government's absolute disregard for the laws of war," said Lama Fakih, senior crisis advisor at Amnesty.
An international commission monitoring the ceasefire in South Sudan has also reported on the incident, while the government has denied it.
Presidential spokesman Ateny Wek Ateny dismissed reports on the killings as "fabricated lies by people working for regime change."