The International Criminal Court is demanding the release of four staff members, including an Australian lawyer, who were detained during a visit to the son of late Libyan leader Colonel Gaddafi.
The group travelled to Libya on 6 June to meet Saif al-Islam in the western town of Zintan where he has been held since his capture last November.
Local militia guarding him claim one of the delegation, Australian lawyer Melinda Taylor, was carrying "dangerous" documents to him.
A spokesperson for the International Criminal Court, Fadi El Abdallah, says it has called on the Libyan authorities to release the four people immediately.
So far, the local militia have ignored government requests to release the delegation, saying "spying and recording" materials had been found and the group members would be further interrogated.
The ICC has previously expressed concern at the conditions under which Saif al-Islam is being held in Libya, Reuters reports.
Libya's interim government has so far refused to hand over Saif al-Islam for trial in the Netherlands where the ICC is based, the BBC says. Libya has insisted he should be tried by a Libyan court.