A U.N. investigation found that at least 85 people were killed and at least 34 others were reported missing following early 2013 conflict. Armed gangs attacked cattle grazers in the area, South Sudan's largest state.
The U.N. report found South Sudan sent military forces to the area, but they were outnumbered. The Office of the U.N. High Commissioner for Human Rights said it was urging restraint given the lingering violence in the area.
"We call on the South Sudanese authorities to immediately launch an investigation into this gruesome attack and bring the perpetrators to justice in order to end the cycle of violence and prevailing impunity," an OHCHR statement read.
South Sudan this week announced that it restarted oil production as part of a peace agreement with Sudan. Plans are reportedly under way to establish a demilitarized zone along their shared border.
South Sudan gained independence from Sudan in 2011 as part of a peace deal that ended a civil war. Conflicts like the raid in Jonglei, oil disputes and border clashes have threatened the peace agreement, however.