At least 10 people have been killed after South Sudanese troops fired on demonstrators angry at officials moving the seat of local authority outside a state capital, the United Nations said Sunday.

Four people were killed in the town of Wau during clashes overnight Saturday, while six were shot dead on Sunday after they gathered to protest the previous night's killings, said UN peacekeeping mission spokesman Liam McDowall.

"A crowd demonstrating the excessive use of force -- in their opinion -- gathered in the town with the intention to go and demonstrate outside the governor's residence... the SPLA opened fire," McDowall said.

There were conflicting reports however as to whether some of the demonstrators had been armed.

"We are investigating the allegations of armed elements inside the demonstrations, as well as allegations of the disproportionate use of force by the army against civilians," McDowall added.

Protests began after officials said they would move the seat of local authority out from Wau, capital of Western Bahr el Ghazal state, to a nearby smaller settlement of Bagare.

Troops were sent in on Saturday to remove protestors blockading roads leading out of Wau, while UN peacekeepers had been shuttling between demonstrators and the army to try to calm both sides.

"A number of protestors fled to the cathedral where they took sanctuary," McDowall said. The army later surrounded the building and had to be persuaded back to their barracks by the Bishop of Wau, he added.

The situation was "still tense" on Sunday and the authorities had issued a curfew from dusk until dawn, McDowall said.

Army spokesman Kella Kueth said the matter was being investigated but could provide no further details.

South Sudan, the world's youngest nation, is awash with weapons after decades of war with Sudan, which it broke free from in July 2011.

Its security forces are made up of former rebel fighters, many who have struggled to integrate into a well-ordered and cohesive force, despite UN-backed efforts to provide training.

A peace deal was signed in 2005 to end decades of civil war -- a key stepping stone towards South Sudan's eventual independence -- but the fledgling nation remains volatile.

Violence is common in the grossly impoverished state, with age-old cattle raids becoming increasingly deadly due to the ready availability of guns and women and children often being killed in what were once attacks to steal livestock.