South Sudanese government troops killed 50 civilians by keeping them in a hot container where they suffocated, a commission monitoring the ceasefire between the government and rebels said.
"About 50 people suffocated in a container on about 22 October" in northern Unity State, the Joint Monitoring and Evaluation Commission (JMEC) said in the report made public at an African Union summit in Addis Ababa late Sunday.
"We don't have that information," presidential spokesman Ateny Wek Ateny told dpa by telephone, calling the allegations "completely ridiculous" and "fabricated lies by people working for regime change."
JMEC said that "the investigation was protracted. Attribution of responsibility: government forces," without giving further details.
The commission also reported other violations of the peace deal signed by President Salva Kiir and rebel leader Riek Machar in August.
They included "rape and murder" by unidentified uniformed men in November, and the killing or wounding of 12 people in the ambush of a civilian vehicle by rebels in December.
The militarized power struggle between Kiir and Machar has killed tens of thousands and displaced more than 2.3 million people since mid-December 2013.
The peace agreement foresees the establishment of a transitional government including government and rebel representatives. But its implementation is "lagging far behind schedule" and the "agreement risks becoming ineffectual," JMEC chairman Festus Mogae said in the report.
The commission accused Kiir of "contradicting the terms of the agreement" by announcing the division of South Sudan into 28 states, up from the previous 10 states.
The rebels want Kiir to cancel the move, which they regard as a form of gerrymandering aimed at diluting their power in the future government.
"The government's action has created an impasse" while "almost half the population is in need of humanitarian assistance" and "ceasefire violations continue to be reported," the commission said.