Joint Monitoring and Evaluation Commission (JMEC) said in a report that South Sudan government troops killed nearly 50 people by stuffing them into a shipping container in the baking heat, noting the latest atrocities in the two-year-old war.
The report was submitted by the JMEC to the African Union (AU) summit and made public late Sunday.
The atrocity, described as "concerning the killing of civilians in Unity State", was one of several listed examples of ceasefire violations by forces on both sides.
"About 50 people suffocated in a container on about October 22. The investigation was protracted. Attribution of responsibility: Government Forces," the report said.
In South Sudan, metal containers are often used as makeshift prison cells. Temperatures in the northern battleground state of Unity regularly reaches 40 degrees Celsius.
Although a peace deal was reached in August, fighting continues and the conflict now involves multiple militia forces, driven by local agendas or revenge attacks, pay little heed to paper peace deals.
Other examples of ceasefire violations include rape and murder and the capture and looting of the United Nations river barges.
The South Sudanese army have not yet responded but spokesman rejected a long list of previous reports detailing accusations of atrocities.
UN rights experts described last month occurrences of "gang-rape, sexual slavery and forced abortion" in South Sudan.
A UN panel of experts also said that South Sudanese 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 appointed to the role by regional bloc IGAD, warned that efforts to force through a unity government had stalled after Kiir nearly tripled the number of regional states, undermining a fundamental pillar of the power-sharing deal.
"The government's action has led to an impasse," Mogae said in an appeal to the AU.
"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.
The government and rebel sides have been accused of committing ethnic massacres, recruiting and killing children and carrying out widespread rape, torture and forces displacement of populations to “cleanse” areas of their opponents.
IGAD bloc called rival forces for allowing food into conflict zones on the verge of famine, where aid workers warned that tens of thousands of people may starve to death.
IGAD said it was "appalled by the deterioration in the humanitarian situation in South Sudan, and demands immediate action by the parties to ensure unconditional humanitarian access."
More than 200,000 South Sudanese civilians are now sheltered inside UN camps surrounded by razor wire.
South Sudanese government negotiator Nhial Deng Nhial dismissed concerns about negotiations stalled due to ongoing violence and fears of potential famine.
"As far as we’re concerned, the implementation of the peace process still remains on track," Nhial said during AU talks on Saturday.