"Three months ago, Siham Sergewa, an elected member of the House of Representatives, was seized at night from her home in Benghazi," in eastern Libya, UNSMIL said in a statement.
"Since her violent abduction by armed men, Ms Sergewa's fate remains unknown."
Three killed in renewed fighting in southern Tripoli: witness
Sergewa disappeared on July 17 in an attack that also allegedly saw her husband shot in the leg, shortly after she spoke on Al-Hadath television - which backs eastern Libyan commander Khalifa Haftar.
During the broadcast she had called for "an end to the bloodshed", referring to an offensive launched in April by Haftar's forces to wrest control of Tripoli from those loyal to a unity government.
UNMSIL said that "relevant authorities in eastern Libya have the legal responsibility to establish the fate and whereabouts of Ms Sergewa".
Haftar, who controls much of eastern Libya including Benghazi, launched his bid to seize the capital Tripoli from the UN-recognised Government of National Accord on April 4.
As Libya's war drags on, Tripoli drowns in waste
UNSMIL said that since the start of the offensive, it had recorded an "alarming increase" enforced disappearances across Libya.
"Ms Sergewa's enforced disappearance sends a message of terror to elected officials over their right to freely express themselves," it said.
Libya has been mired in chaos since a NATO-backed uprising that toppled and killed dictator Moamer Kadhafi in 2011.
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