Thirteen United Nations mission staff have been taken hostage by 100 unarmed South Sudanese refugees in the Democratic Republic of Congo, demanding to be moved to a third country, according to a UN official based in the area.

The 100 were among 530 people who have been living in the Munigi base, outside Goma, since fleeing South Sudan last August, UN Goma bureau head, Daniel Ruiz, told the Reuters news agency.

An official, who asked not to be named, said the UN was negotiating to try to win the release of the employees from the MONUSCO mission.

"We can confirm that some MONUSCO staff members are currently being held in a camp for former combatants in Munigi in eastern [Congo]," said the official. "The mission is working to resolve the situation."

Most of the captors are former fighters loyal to ex-Vice President Riek Machar, who have clashed with President Salva Kiir's forces since July 2016.

The UN estimates about three million South Sudanese have been uprooted by the violence in their country, the biggest cross-border exodus in Africa since the 1994 Rwandan genocide.

Ruiz said the camp occupants had been demanding to be moved to a third country for months, but no one would take them.

On Friday, eight of them agreed to be repatriated to South Sudan's capital, Juba. Others fear going back and are frustrated at being confined in the tiny camp in Congo.

"They're [the captors] saying if the eight were transferred to South Sudan, why shouldn't we be able to go to a third country?" Ruiz said. He added that the UN mission was currently negotiating with them.

Civil war broke out in South Sudan in 2013, after Kiir sacked Machar from the vice presidency.

That conflict ended in a peace pact in 2015 and Machar was reinstated early last year, but tensions between the two men lingered and finally erupted into new fighting in July.