"What I saw was a horror," he said in a statement at the conclusion of his four-day visit to the country. "The extent of the looting, burning and destruction is hard to grasp for anybody who hasn't been there."
Violence erupted in South Sudan in December when President Salva Kiir, a member of the Dinka ethnic community, accused former Vice President Riek Machar, a member of the Nuer community, of trying to overthrow the government. Machar was sidelined when Kiir reshuffled his Cabinet in July, but denies waging a coup.
Representatives from both sides are working in neighboring Ethiopia to find a peaceful resolution to the situation.
Simonovic said "mass atrocities" have been committed by both sides.
"The conflict has now reached the threshold of an internal armed conflict, causing untold suffering for thousands of civilians," he said.
The rights envoy said "thousands" of people have died as a result of violence that erupted in mid-December. An estimated 70,000 people are seeking protection at U.N. camps set up throughout South Sudan.