Findings compiled by the U.N. Mission in South Sudan and the Office of the High Commissioner for Human Rights were released in a report Monday, documenting the crimes and incidents between the Murle and Lou Nuer ethnic groups in South Sudan's Jonglei state in 2011 and 2012, the statement said.
The report says 6,000 to 8,000 armed youths of the Lou Nuer group launched a 12-day attack on the Murle, who organized and retaliated. The violence led to deaths, abductions of women and children, destruction of homes and the displacement of thousands of people.
The report called for immediate government action, including the prosecution of those responsible and activation of a government investigation, the statement said.
"To end the cycle of violence in Jonglei for good, we need to understand what happened and make sure that the perpetrators are held accountable," she said in a statement. "This needs to be part of a comprehensive peace process in the state."
U.N. delegates visited regional areas afflicted by conflict alongside members of the Sudan People's Liberation Army, South Sudan's military. The agency said their help was indispensable, but delays in SPLA intervention left many civilians vulnerable.
South Sudan gained independence last year under the terms of a peace agreement with Khartoum. Oil disputes, border conflicts and ethnic tensions are threatening the deal, however.