Nine protesting civilians were killed by security forces.
South Sudan’s government had been struggling to reform its bloated security services since it split from Sudan in July 2011 after a long civil war that left the new country awash with weapons.
Human rights groups regularly accuse South Sudan’s Army, an assortment of poorly-trained former guerrilla fighters known as the SPLA, of abuses against civilians.
At least 2,400 people had been killed in rebel and tribal violence since independence, the UN said.
The SPLA said police and soldiers opened fire on civilians who were protesting against the relocation of local council offices at the weekend.
South Sudan’s Civil Society Alliance, an umbrella group for local civil society organizations, quoted witnesses as saying the demonstrators had been unarmed and protesting peacefully.
“UNMISS (United Nations Mission in the Republic of South Sudan) is investigating the use of force and other reports of violence in order to establish the facts behind these tragic events,” UNMISS said in a statement.
“Nine civilians had been confirmed killed and many more injured in incidents involving demonstrators and members of the security forces,” it added.
The violence in Western Bahr el Ghazal state’s capital, Wau, began on December 8 when the army was sent to dismantle road blocks set up by civilians protesting against plans to move the municipal council headquarters outside Wau.
The following day, security forces fired on protesters marching toward government offices with a petition asking the governor to reverse the decision.
SPLA spokesman, Kella Kueth, said the protesters burnt nine trucks.
“They were thought to be destroying the government property and that is when they were fired upon by police and some army forces,” Mr. Kueth told Reuters.
Ateny Wek, spokesman for the Civil Society Alliance, said two more people died from their injuries, bringing the death toll to eleven, with scores of others injured.