UN officials were negotiating to try to win the release of the employees from the MONUSCO mission in DR Congo, said the official, who asked not to be named.
"We can confirm that some MONUSCO staff members are currently being held in a camp for former combatants in Munigi in eastern DRC," said the official.
"The mission is working to resolve the situation."
About 530 former rebels from South Sudan are living in the Munigi camp, just outside of Goma, after fleeing across the border when fighting flared in the South Sudanese capital of Juba in July.
The combatants, who were disarmed when they entered the camp, have for months demanded that they be relocated, but UN officials have been unable to find countries willing to take them in.
Rebel leader Riek Machar remains in South Africa and has been prevented from returning to South Sudan.
UN officials declined to give details about the nationalities of the UN staff being held, citing security concerns.
After gaining independence from Sudan in 2011, South Sudan descended into war in December 2013, leaving tens of thousands dead and 3.5 million people displaced.
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