At least 14 people have been killed in South Sudan after new fighting erupted between government forces and the main rebel group.

Lam Paul Gabriel Lam, the SPLM-IO rebel group's spokesman, told Reuters news agency on Saturday during the past two days the army has bombed rebel-held areas around Raga - a northwestern town near the border with Sudan and Central African Republic.

"Yesterday our forces decided to go and raid Raga," he said. "Around 14 people were counted killed but many are injured. We had one soldier killed with some injuries."

Santo Domic Chol, the government's military spokesman, said he was in Raga and would provide information later on.

The United Nations, which has a large peacekeeping force in the East African nation, confirmed the latest outbreak of violence.

"Fresh fighting has broken out between government and opposition forces in a number of locations including Raga in the west of South Sudan, Waat in Jonglei to the east, and in the area of Wunkur and Tonga in the northern Upper Nile region," the UN said in a statement.

Aid workers flee

The surge in fighting in eastern South Sudan has forced 60 aid workers to flee, hurting efforts to help desperate civilians in the famine-hit nation, the UN humanitarian agency OCHA said on Saturday,.

Three UN World Food Programme workers were killed this week while trying to get to a supply warehouse amid fighting between rebel and government troops near the western city of Wau.

"There are no words left to explain the level of frustration and outrage I feel regarding the continued attacks against humanitarians in South Sudan who are simply trying to help the civilians suffering as a result of this conflict," said Eugene Owusu, OCHA coordinator, in a statement.

"Separately, 60 humanitarian workers have had to relocate from multiple locations in Jonglei yesterday and today - including Waat and Walgak - due to intensified conflict in the area."

The outbreak of violence in the eastern Jonglei region came after fierce clashes in southern Pajok and western Wau in the past two weeks.

According to a tally of UN figures and deaths that witnesses reported to AFP, 101 people were killed in the two towns.

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The UN peacekeeping mission UNMISS, which has been blocked from accessing some conflict zones, said 13,500 people had fled to their base near Wau this week. More than 3,000 others were seeking refuge at a Catholic church compound.

Moustapha Soumare, the special representative for the UN secretary general in South Sudan, on Saturday called for a halt to fighting, urging "all parties to prove their commitment to peace".

"They must show restraint and demonstrate their responsibility to ensure the sanctity of life of all South Sudanese citizens," he said in a statement.

Oil-rich South Sudan has been riven by violence since 2013, when President Salva Kiir, an ethnic Dinka, fired his deputy Riek Machar, an ethnic Nuer.

The political showdown quickly split along ethnic lines and has drawn many tribes into a complex patchwork of conflict.