18 Bangladeshi peacemakers have been released from captivity by the South Sudan rebels.
However, 12 United Nations contractors who were travelling with them on the Nile Rivers are still imprisoned by the rebels.
On Monday, over 100 heavily-armed rebels detained the peacekeepers along with the crew of a barge carrying fuel for the UN mission, north of Malakal, the Daily Star reported.
They were later taken by the rebels to the town of Kaka, in Upper Nile state, where the contractors have been kept.
According to spokesman Stephane Dujarric, UN has called on the rebel leader Riek Machar to uphold his commitment to the organisation by immediately releasing all of the crew members unharmed and unhurt.
Attack against UN peacekeepers is punishable as a war crime, he added.