UNITED NATIONS - One civilian was killed and dozens, including a UN military officer, were wounded during an attack on a UN base in northern South Sudan, a UN spokesperson said Wednesday.

Citing reports from the UN Mission in South Sudan (UNMISS), UN spokesperson Martin Nesirky said situation in Malakal, capital of the Upper Nile state, was in flux after its base in the city was assaulted on Tuesday.

One civilian seeking shelter at the base was killed and dozens, suffering gunshot wounds, have received medical attention at the mission's hospital, he said.

"The mission said that peacekeepers fired multiple rounds of gunfire to deter anyone engaged in fighting from coming too close to its base, in order to protect civilians," said Nesirky, adding some 20,000 civilians continue to seek refuge at the compound.

Although the fighting has ceased, sporadic gunfire could be heard in Malakal and near its base in Bor, which is housing 9,000 civilians. An additional 6,000 civilians are seeking shelter within the UN base in Bentiu.

Clashes broke out in South Sudan since mid-December between military factions loyal to South Sudan President Salva Kiir and forces backing former Vice-President Riek Machar.

The month-long conflict have driven more than 400,000 civilians away from their homes. Last week alone, 78,000 people fled to neighboring countries, according to UN figures.

The fighting has escalated even as the parties are negotiating in the Ethiopian capital Addis Ababa under the patronage of the Inter-Governmental Authority for Development. The two sides have failed so far to reach a cease-fire agreement.

UN Secretary-General Ban Ki-moon called Tuesday on all parties to cease hostilities and urged them to engage constructively with the negotiation process.