Representatives of the International Criminal Court arrived in Tripoli on Sunday to try to secure the release of a detained delegation visiting Muammar Gaddafi's captured son, a Libyan official said.
The four-member delegation was being held in the western mountain town of Zintan after one of its lawyers, Australian Melinda Taylor, was found carrying documents regarded as suspicious for Saif al-Islam Gaddafi, a Libyan lawyer and a militia member said on Saturday.
The president of the international war crimes court demanded their immediate release.
"An (ICC) delegation arrived today in Tripoli. They are holding meetings with officials about this," said the Libyan official, without giving further details.
Reflecting Libya's wider problem of powerful local militias and a weak central government, the Zintan brigade holding Saif al-Islam said it would not heed the government's request to release the four ICC staff before questioning them.
"They are still under investigation," a member of the brigade said. "The visiting delegation won't see them just yet."
Australian Foreign Minister Bob Carr said he had spoken to Libyan Deputy Foreign Minister Muhamed Aziz about Melinda Taylor's detention.
He said Aziz had confirmed in a telephone call that Taylor was "being held by Libyan authorities in Zintan and would be detained pending further inquiries," Carr said in a statement.
"I raised Australia's concern for Ms Taylor's welfare and Mr Aziz assured me that she is safe and well," Carr said in the statement. "I emphasised our strong interest in seeing the matter resolved quickly and urged Mr Aziz to facilitate full consular access to Ms Taylor."
Carr said he had also spoken to Taylor's husband and to the president of the International Criminal Court.
Saif al-Islam, held in Zintan since his desert capture in November, is wanted by the ICC for crimes an uprising last year that ended his father's 42-year rule. Libya's new rulers insist he should be tried in his home country.
The ICC has previously expressed concern at the conditions under which he is being held. Human rights groups also question whether Libya's justice system can meet the standards of international law.
A Libyan lawyer said the suspicious documents included letters from Saif al-Islam's former right-hand man Mohammed Ismail, as well as blank documents signed by the prisoner.
The international court said the 36-year old Taylor has been working at the ICC since 2006 as counsel in the office that represents ICC indictees' interests before the appointment of a formal defence counsel.
The ICC named the three other staff members as Helene Assaf, an ICC translator and interpreter since 2005; Esteban Peralta Losilla, the chief of the Counsel Support Section at the ICC; and Alexander Khodakov, a Russian career diplomat who is the external relations and cooperation senior adviser at the registry of the ICC.