"The arrest and detention by the Tunisian authorities of Moncef Kartas... while he was performing his official duties is a matter of very grave concern," the spokesman for the UN secretary-general said in a statement Friday.
"The continued detention is in violation of the privileges and immunities that Mr Kartas enjoys," he added.
The UN has engaged with Tunisia's government "at the highest levels", according to the spokesman, making clear its legal position through four separate legal notes.
But the government had "failed to provide an adequate response", he said.
Tunisia's interior ministry said last month "confidential documents containing sensitive detailed data capable of harming national security" were seized in relation to UN expert's arrest.
It also alleged technical equipment for jamming and intercepting communications -- banned in Tunisia -- had been confiscated.
The panel reports to the UN sanctions committee on Libya and details its findings in an annual report.
An interim report is due in June.
Previous reports by the panel have found that arms and ammunition have repeatedly been delivered to warring parties in Libya in violation of the UN embargo, with the involvement of member states.
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