"Hostilities have this morning broken out in Malakal [in the north of the country]." U.N. Humanitarian Coordinator for South Sudan Toby Lanzar said in a statement posted from his Twitter account. "All parties engaged in the violence must uphold people's rights and protect non-combatants."
Violence erupted in South Sudan in December when President Salva Kiir, a member of the Dinka ethnic community, accused former Vice President Riek Machar, a member of the Nuer community, of trying to overthrow the government. Machar was sidelined when Kiir reshuffled his Cabinet in July, but denies waging a coup.
Kiir's spokesman, Ateny Wek Ateny, told the BBC "well-equipped" rebel forces attacked their positions, though rival parties said it was the government that attacked first with the help of Ugandan troops.
Representatives from warring sides in South Sudan met last week for talks in Ethiopia with mediators from the region's Intergovernmental Authority on Development to try to resolve the crisis.
Herve Ladsous, U.N. undersecretary-general for peacekeeping operations, said last week the road to recovery for South Sudan would be "very long."