The Iraqi employee, whose name has not been released, was seized on Sunday in Diyala's provincial capital Baquba, Eliana Nabaa said yesterday.
"We are coordinating with the government" but further details on the incident are not yet available, she said.
"We hope we will have this colleague back among us very soon."
The abduction comes at a time when hundreds of thousands of Iraqis displaced by violence over the past year are depending on UN assistance to survive.
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But the threat has been on the rise again since last year, when the Islamic State jihadist group led an offensive that overran large areas north and west of Baghdad.
IS has kidnapped thousands of people in Iraq, and Baghdad turned to Shiite militias with a history of kidnappings and other abuses to help bolster its forces, increasing the threat further.
The UN suffered one of the worst attacks in its history in Iraq in 2003 when a suicide bomber detonated an explosives-rigged truck at the Canal Hotel in the Iraqi capital.