"It is clear that the conflict has taken a dangerous trajectory, and civilians are being deliberately targeted based on their ethnicity and perceived political affiliation," added Dieng, who has served on the International Criminal Tribunal for Rwanda.
Thousands have been killed in the brutal four-month civil war, and more than a million have fled their homes.
"South Sudan should not be led down this slippery slope," Dieng said.
"I beseech everyone; the South Sudanese, your leaders, the regional and international community, to take immediate measures to end the violence and uphold our collective responsibility to protect the populations of South Sudan from genocide, war crimes, ethnic cleansing and crimes against humanity."
UN bases have come under attack, and peacekeepers struggled to contained the waves of violence.
UN Secretary General Ban Ki-moon is "strongly concerned, and will make sure that never again what happened in Rwanda happens in another place in this continent," Dieng said, adding that "the world is watching."