Assistant Secretary-General for Human Rights Ivan Simonovic's comments came Friday at the end of his four-day visit to South Sudan, the United Nations said in a release.
"Mass atrocities have been committed by both sides. During my visit, I have received reports of mass killings, extrajudicial killings, arbitrary detention, enforced disappearances, sexual violence, the widespread destruction of property and the use of children in the conflict," Simonovic said.
"One month of conflict has set South Sudan back a decade," he said. "Thousands of people have been killed and hundreds of thousands are now displaced, with some 70,000 people seeking protection in U.N. camps and 30,000 in the two U.N. compounds in Juba alone."
The U.N. Refugee Agency said crews from the United Nations High Commissioner for Refugees are building new refugee camps and expanding existing ones in Uganda, Ethiopia and Kenya, to help those countries cope with an influx of refugees from South Sudan, the UNHCR said in a release.
"With people still arriving at a rate of around 1,000 a day, we are looking at the prospect of refugee numbers exceeding 100,000 by the end of January," UNHCR spokesman Adrian Edwards said.
"UNHCR is racing to transfer around 500 families daily to decongest Dzaipi transit center as more refugees arrive. We are meanwhile rehabilitating former sites in Nyumanzi and nearby at Baratuku where a primary school and health center need to be upgraded," Edwards said.
Clashes in South Sudan began Dec. 15 after President Salva Kiir accused former Vice President Riek Machar, whom he dismissed in July, of attempting a coup. Machar has denied plotting to overthrow Kiir.
The conflict has also taken on ethnic undertones, as Kiir is from the Dinka community and Machar from the Nuer group, the BBC said.