A rights group says it has evidence that South Sudan government forces deliberately suffocated more than 60 men and boys in a shipping container.
They then dumped the bodies in a field in Leer Town, Unity State, Amnesty International said.
In a separate report, the UN has also accused the government troops of deliberately killing and raping civilians.
The government denies its army targeted civilians but says it is investigating.
“We have rules of engagement and we are following them,” a spokesman for President Salva Kiir, Ateny Wek Ateny, told the BBC.
Researchers from Amnesty International said they had found the remains of skeletons from the killings, which are are said to have taken place last October.
Thousands of people have been killed and millions displaced since 2013.
“Dozens of people suffered a slow and agonizing death at the hands of government forces that should have been protecting them. These unlawful killings must be investigated,” said Lama Fakih from Amnesty.
Amnesty researchers interviewed more than 42 witnesses, including 23 people who said they saw the men and boys being forced into a shipping container and later saw their bodies either being removed or at a mass burial site.
Witnesses described hearing the detainees screaming and banging on the walls of the shipping container, which they said had no windows or other form of ventilation.
One said she saw a commander order soldiers to open the container and take out four bodies before closing it again on the remaining detainees.
Relatives of the victims told Amnesty that the victims were cattle keepers, traders and students, not fighters.
Amnesty has called for the African Union (AU) to set up a Hybrid Court for South Sudan agreed under a peace agreement signed last August.