Fighting was continuing across South Sudan on Sunday, the army said, even as peace talks between the government and rebels were set to begin in neighbouring Ethiopia.

Army spokesman Philip Aguer reported ongoing clashes in the oil-producing Unity and Upper Nile states in the north, saying that government forces were advancing on the two state capitals of Bentiu and Malakal, currently in rebel hands.

Government troops were preparing to retake Bor, capital of Jonglei State, he added.

"The SPLA (Sudan People's Liberation Army) forces are advancing from the northern part of Bentiu. We will try to do our constitutional duty... sooner or later our target is Bentiu," Aguer told reporters.

"It's matter of time, our forces are advancing towards Bor," he said, claiming that the rebels "realise they are fighting a useless war" and saying government forces were a mere 15 kilometres (nine miles) from the town.

There was no immediate comment from the rebels.

The spokesman admitted that an army unit in the town of Yei, situated south of Juba and near the border with Uganda, had defected to the rebels on Saturday and left the area in a number of vehicles.

He also reported another defection in Western Equatoria State, an area which has so far largely escaped the now-three-week-old outbreak of fighting.

Still, Aguer assured that the government was "in control of most of the parts of the country", repeating that "the situation is under control".

"We are confidently telling the public that South Sudan is relatively stable," he said.

The conflict in South Sudan erupted on December 15, pitting army units loyal to President Salva Kiir against a loose alliance of ethnic militia forces and mutinous army commanders nominally headed by Riek Machar, a former vice president who was sacked last July.

Talks between the two sides on a possible ceasefire were set to open in the Ethiopian capital Addis Ababa on Sunday.