Libya on Monday freed four envoys of the International Criminal Court who had been detained after visiting a son of slain dictator Mummar Gaddafi last month, officials said.

"The four members (of the ICC team) were released," Ajmi al-Atiri, commander of a brigade holding Seif al-Islam Gaddafi, told journalists in Zintan, southwest of Tripoli.

The four, including Australian lawyer Melinda Taylor, had been held in Zintan since June 7 after travelling to the hilltop town to help prepare Seif al-Islam's defence.

Taylor was accused of carrying a pen camera and attempting to give Seif al-Islam a coded letter from his former right-hand man, Mohammed Ismail, who is wanted by the Libyan authorities.

The other three detained ICC staffers are Taylor's interpreter from Lebanon, Helen Assaf, and two colleagues, Russian Alexander Khodakov and Esteban Peralta Losilla from Spain.

Libyan Deputy Foreign Minister Mohammed Abdel Aziz said the ICC envoys would leave the North African country later on Monday, as part of an agreement that his government reached with The Hague-based court.

ICC president Sang-Hyung Song, speaking at the same news conference in Zintan, thanked the Libyan authorities for mediating the release of the court's legal team.

"The ICC is grateful to the Libyan authorities for making the necessary arrangements to allow the release of the ICC members," Song said.

"I wish to apologise for the difficulties which arose due to these events. There will be an investigation following the return of (ICC) members to The Hague. Any member found with any misconduct will face appropriate sanction."

Following the announcement, a late lunch was served for South Korea's Song and the newly released ICC envoys who appeared to be in good health, an AFP journalist at the scene said.

Taylor and Assaf were dressed in black abayas, a loose cloak worn by some women in parts of the Muslim world, and matching Islamic headscarves, the reporter added.

The ordeal of the ICC legal team might not be over just yet.

Abdel Aziz said the four had been summoned to a Libyan court on July 23 to complete the judicial process which was set in motion by the prosecutor general's investigations.

Any verdict would be transfered by the ICC for follow-up, he said, adding: "The ICC has agreed to keep Libyan authorities informed on the results of its investigations."

It was unclear whether they would have to return to Libya for the case or be tried in absentia.

Libyan officials have said that the actions of the ICC team represented a "breach of national security," a matter Tripoli did not take lightly, despite its declared commitment to cooperate with the court.

Ajmi said on Monday that he had anticipated there would be attempts to free Seif al-Islam by supporters of the former regime but not by members of a renowned international institution.

"What is regrettable is that these acts were carried out by an international institution which we respect and that played an important role during the Libyan revolution," he said.

The ICC wants to try Seif al-Islam, 40, for crimes against humanity allegedly committed last year when his father's regime attempted in vain to crush a popular uprising.

Tripoli insists he should be tried before a Libyan court and filed a motion on May 1 challenging the jurisdiction of the International Criminal Court.

But it has failed to secure the prisoner's transfer to Tripoli from Zintan, where has been held since his capture last November.