At least 50 people suffocated to death when they were shoved into metal containers in sweltering conditions by troops belonging to South Sudan, a commission monitoring the ceasefire between rival factions said.
The incident was one of several listed as examples of ceasefire violations carried out by forces on both sides, outlined in a report compiled by the Joint Monitoring and Evaluation Commission (JMEC), which was made public at the African Union (AU) summit on Sunday.
"About 50 people suffocated in a container on about October 22. The investigation was protracted. Attribution of responsibility: Government Forces," the report said in a section titled "concerning the killing of civilians in Unity State".
Speaking to Al Jazeera, Angelos Agok, director in the office of the chief of general staff of the army, denied that the incident happened.
Report: Mass graves, rape and cannibalism in South Sudan
Despite an August peace deal, fighting continues in the country, and the conflict now involves multiple militias who according to the AU report are driven by local agendas and revenge attacks.
In South Sudan, metal containers are often used as makeshift prison cells. Temperatures in the northern battleground state of Unity regularly top 40 degrees Celsius.
Last month, a United Nations panel of experts said South Sudan's President Salva Kiir and rebel chief Riek Machar should face sanctions for their role in the war.
JMEC chief Festus Mogae, a former Botswana president, said the AU had a duty to respond to the allegations laid out in the report.
"Given the fragility of the security situation in South Sudan, the renewed risk of conflict, and continued insecurity affecting the humanitarian relief effort, emphatic, stern measures should be taken by the African Union ... rhetoric alone can only do so much," Mogae said.
Both the government and rebel sides have been accused of perpetrating ethnic massacres, recruiting and killing children and carrying out widespread rape, torture and forced displacement of populations to "cleanse" areas of their opponents.