The United Nations says 16 staff members held hostage at a UN camp in the eastern Democratic Republic of the Congo (DRC) have been released unharmed.

The UN peacekeeping department said in a statement late on Tuesday that "the camp is quiet and under full control" of its peacekeepers.

Unarmed South Sudanese refugees took the staff members hostage earlier on Tuesday at the Munigi base in Congo's North Kivu province, demanding they be sent to another East African country to avoid their forced return to South Sudan, the UN Mission in Congo said.

Negotiations had continued into the evening.

The UN peacekeeping department said: "All staff have returned safely to their homes. No casualties have been reported."

It said the mission is investigating the incident.

The captors 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.

Most of them 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 DRC.

"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.

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.