The South Sudanese army killed 10 people when it opened fire on demonstrators angry at the relocation of a local council HQ in the north-west of the country, the UN says.
A United Nations spokesman said the troops had demonstrated "excessive use of force" in breaking up protests on Saturday night, when four people died.
Six more people died when troops fired on a further demonstration on Sunday.
The army said it had launched an investigation.
A spokesman said it was examining whether disproportionate force had been used.
The spokesman also said the army was investigating reports that some of the protesters had been armed.
Demonstrations had begun when local government officials said they would relocate from Wau, capital of Western Bahr el Ghazal state.
In protest, residents blocked roads out of Wau and the army was sent in to relieve the blockade.
The United Nations said it had been trying to negotiate between the two sides.
Human rights groups regularly accuse South Sudan's army, a collection of poorly trained former guerrilla fighters and militiamen, of human rights abuses.
A dusk to dawn curfew has now been imposed in Wau.