Amnesty International documents deliberate targeting of civilians by fighters aligned with rival political factions.
The two-day clashes erupted over the weekend after some armed youths in the Greater Tonj area engaged with security forces.
Clashes are taking place between South Sudanese government forces and rebels in Pagak, rebel spokesman says.
The UN High Commission for Refugees releases annual survey on the global refugee situation. Years of war in Syria, conflicts in South Sudan and elsewhere have pushed the number of refugees around the world to an all-time high.
The UN says at least 16 civilians were killed and 10 injured in ethnic clashes between pro-government militia and rebel groups in the northwestern city of Wau.
A senior government official on Monday said nearly half the country's population would be short of food by July. Last week, the UN said the number of refugees who have fled the fighting in South Sudan has surpassed 1.5 million.
The latest report from the World Food Programme says the number of people facing severe hunger is expected to rise to 4.6 million between January and April next year and increase even more from May to July unless aid is scaled up.
At least 120 such cases recorded against civilians in the capital, Juba, since fighting erupted three weeks ago. Fear of a return of civil war in South Sudan simmers after President Salva Kiir replaces Vice President Riek Machar.
With reports of divisions within both warring camps, the fighting has fragmented the world's youngest country and multiple militia forces now pay no heed to either President Kiir or Vice President Machar.
US says it will impose sanctions and arms embargo on South Sudan if leaders backtrack and do not end the conflict
South Sudan rebel chief Riek Machar returns to Juba to join a unity government formed after more than two years of civil war.
South Sudan rebel leader Riek Machar misses international deadline to return to South Sudan's capital Juba
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