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.
At least 16 civilians were killed and thousands were displaced on Monday in ethnic clashes between government forces and rebels in South Sudan's second-largest city of Wau, the United Nations (UN) peacekeeping mission, UNMISS, said in a statement.
The violence stemmed from an ambush on Sunday of government troops near the city, leading to clashes in Wau the following day, UNMISS said.
"The mission mounted two patrols into Wau on Monday and said it had observed the bodies of 16 civilians in a hospital. There were ten people who had been injured," the statement said.
According to the peacekeeping mission some 3,000 people — mostly women and children — had fled to a Catholic church in the town.
Another 84 had sought refuge at an UNMISS Protection of Civilians site.
More than 200,000 people have taken refuge in such sites set up across the country after widespread ethnic killings, many by soldiers.
Witnesses said ethnic militiamen aligned with the government in the country's ethnically charged civil war went house to house searching for people from other groups.
They accused army soldiers of blocking the main road to a civilian encampment protected by UN peacekeepers. Some reported seeing killings.
"The [people] who came are reporting to us that there are SPLA (national army) soldiers in the residential areas," a local priest Moses Peter said.
Wau is located in a region that has repeatedly changed hands between government troops and rebels loyal to former vice president Riek Machar since the country descended into civil war in 2013.
The civilians were "killed because they are suspected of supporting rebels," local resident Tibur Erynio, 41, said, adding that this was not true.
Erynio said shops and markets in the city's southern half were closed and the government had told people to stay indoors.
Campaign groups have accused both sides of atrocities.
The 15,000-strong UN peacekeeping mission, which has a base in Wau, has not been able to stop the killing.
"We are aware of the situation in the town and we are looking into it," UN spokesman Daniel Dickinson said.
The military and rebels gave conflicting accounts of the violence.
Military spokesperson Colonel Santo Domic Chol said the fighting started as government troops were trying to rout rebels from strongholds in the countryside.
Forces loyal to Machar took cover among civilians when the skirmishes reached Wau town on Monday, Domic said.
Four prison guards were killed in the fighting, Domic said, without elaborating.
Spokesperson for the rebels William Gatjiath Deng said the fighting stemmed from an ambush laid by the rebels outside of Wau on Sunday that left 35 soldiers from a government-aligned militia dead.
The surviving pro-regime forces returned to Wau, where Deng said they killed 50 civilians in "house-to-house" raids.
Both accounts were impossible to verify.
Warfare and famine
South Sudan's leaders fought for decades for independence, but once they got it in 2011, civil war erupted in 2013 out of a power struggle between President Salva Kiir and his former deputy Riek Machar.
Fighting since then has often split the oil-producing country along ethnic lines and created a patchwork of armed factions, and has persisted despite an August 2015 peace deal intended to end the war.
At least 1.7 million people have fled the country because of the war, and 1.9 million are internally displaced.
The war has also created a man-made famine in parts of the country, exacerbating a dire humanitarian crisis.