TUNIS (Reuters) - Tunisia has arrested a U.N. official involved in monitoring an international arms embargo on Libya although he has diplomatic immunity, a U.N. spokesman said on Friday.

Moncef Kartas, a Tunisian national and member of a U.N. panel of experts assigned to monitor an arms embargo imposed on Libya in 2011, was arrested after arriving at Tunis airport on Tuesday, U.N. spokesman Farhan Haq said.

“We are in touch with the Tunisian authorities to ascertain the reasons for his arrest and detention as well as the conditions under which he is being held,” Haq said.

The Tunisian interior ministry could not be reached for comment.

The interior ministry said in a statement on Friday that two unnamed Tunisian citizens were arrested on Tuesday on suspicion of spying for foreign parties. The ministry had also seized secret documents undermining national security, it said, adding that investigations were ongoing.

It was not clear if one of those detained was Kartas.