TUNIS (Reuters) - Tunisian authorities have detained a U.N. official involved in monitoring an international arms embargo on Libya as part of an investigation by an anti-terrorism unit, state news agency TAP said on Saturday.

It was the first time Tunisia had confirmed the arrest of the official, Tunisian national Moncef Kartas, who a U.N. spokesman said on Friday had been detained earlier in the week at Tunis airport despite having diplomatic immunity.

A spokesman for a Tunisian anti-terrorism judiciary unit told TAP the detained official had traveled in a private capacity on his Tunisian passport, meaning he was not entitled to diplomatic immunity and could be arrested.

The U.N. spokesman had also called on Tunisia to clarify the reason for his arrest. Saturday’s TAP report said it was linked to a probe by the special judiciary unit, but gave no further details.

On Friday, the interior ministry said two unnamed Tunisians had been arrested on Tuesday on suspicion of spying. It did not say if Kartas was one of the two.