During a four-day visit to South Sudan, Simonovic said in a statement that he received reports of mass killings, sexual violence and the use of children in conflict.
Describing the situation as a "horror", Simonovic said, "Destruction and death is everywhere in Bentiu, which has now become a ghost town."
"I myself saw some 15 bodies lying on a road. The extent of the looting, burning and destruction is hard to grasp for anybody who hasn't been there. The conflict has taken the country a decade back," Simonovic added.
Stating that the worst affected areas were around Juba, the capital, Bor in Jonglei state and Bentiu, UN official said that the both sides in the country are responsible for the violence.
Simonovic called for an independent fact-finding mission to determine those responsible for the killings.
"Those who committed these terrible crimes, who ordered them or those who did nothing to prevent them while they were in a position to do so, all these people should be held accountable without delay," he said.
The violence broke out on Dec. 15 with gunshots at a meeting of the ruling party in Juba. President Salva Kiir accused his former vice president, Riek Machar, of trying to stage a coup, a charge Machar denies.
The dispute escalated into clashes between members of Kiir’s ethnic Dinka community and Machar’s Nuer group.
From the beginning of the clashes 355,000 people have fled their homes, according to the UN. About 66,900 people are sheltering in UN bases across South Sudan.
The local sources claim that the number of dead from the conflict was close to 10,000 people, a major increase from earlier estimates by the United Nations.
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