Around 100 rebel fighters, who have been battling the government for almost two years, seized 31 members of the U.N. peacekeeping mission in South Sudan (UNMISS) last week.
While 18 Bangladeshi peacekeepers were freed soon after they were held, the 13 remaining UN workers -- all South Sudanese nationals -- were released by the rebels Sunday, UNMISS said Monday, AFP reported.
The UN had last week said only 12 were still being held. However, it said 13 were released Sunday.
All were on a river barge carrying fuel for the UN mission. While the barge was also released, the UN said that rebels had stolen the 55,000 liters of fuel it was carrying, as well as communications equipment, an inflatable boat, and seven weapons.
UN chief in South Sudan Ellen Margrethe Loj said she was "relieved by the safe release of all UN personnel."
She had previously warned that the kidnap "may constitute a war crime."
The UN had last week said only 12 hostages were still being held. However, it said 13 were released Sunday.
Some 12,500 peacekeepers are deployed in South Sudan, which has been wracked by conflict since December 2013.
Tens of thousands of people have been killed, and UN-backed experts have warned of the "concrete risk of famine" before the end of the year, if fighting continues and aid does not reach the hardest hit areas.
Both sides are accused of having perpetrated ethnic massacres, recruited and killed children and carried out widespread rape, torture and forced displacement of populations to "cleanse" areas of their opponents.
Some 3.9 million people are in crisis -- a third of the country's population -- a massive 80 percent rise compared to the same period last year, the UN said.
The army and rebels have repeatedly accused each other of breaking an internationally-brokered August 26 ceasefire, the eighth such agreement aimed at ending the nearly two-year long war.