Libya has arrested an Australian lawyer and three other members of a delegation from the International Criminal Court for allegedly trying to pass "dangerous" documents to Saif al-Islam, the detained son of slain leader Muammar Gaddafi.
"During a visit [to Saif on Friday], the lawyer tried to deliver documents to the accused; documents that have nothing to do with his case and that represent a danger to the security of Libya," said Ahmed al-Jehani, the Libyan lawyer in charge of the Saif al-Islam case and who liaises between the government and the Hague-based ICC.
The lawyer, named as Melinda Taylor, was part of a four-member ICC delegation that received permission from Libya's chief prosecutor to visit Saif in Zintan, southwest of Tripoli, where he is being held.
Jehani said she is "under house arrest in Zintan, not in prison," and is being questioned by the authorities.
The court demanded the delegation's immediate release in a statement on Saturday. "We are very concerned about the safety of our staff in the absence of any contact with them," said Sang-Hyun Song, the court's president.
"These four international civil servants have immunity when on an official ICC mission."
Ajmi al-Atiri, the commander of the brigade that arrested Saif, confirmed at a news conference in Zintan on Saturday that there had been "a security breach" during the ICC visit, and that the lawyer had been detained for questioning.
He said the delegation came on Thursday and that Taylor visited Saif with a female interpreter, who had claimed to be a nurse and was found to be in possession of a camera when searched by a policewomen. He also said there was an attempt to exchange documents, including a letter from Saif accusing the militia of mistreating him.
Al-Atiri said he was "surprised" that Mustafa Abdel Jalil, the head of the National Transitional Council, had called for the immediate release of the detainees.
Meanwhile, a foreign ministry official said that Libya would request that the ICC waive the lawyer's immunity so a formal investigation could begin.
"I think the woman will be with us for a while, until the waiver is granted," Mohammed Abdulaziz told AFP. "It is an act that is jeopardising the national security of Libya, and we are taking it very seriously."
Saif, 39, has been in custody in Zintan since his arrest on November 19 in the wake of the uprising that toppled his father's rule after more than 40 years in power.
Al Jazeera's Omar al-Saleh, reporting from Libya's capital, said it was "highly unlikely that the Zintan brigade would hand over Saif to be tried in Tripoli".
The reason, he said, was because "the captors and the people of Zintan want to try him locally. It's their fighters that caught him after all, and they pride themselves with that fact".
The ICC wants both Saif and his late father's intelligence chief, Abdullah Senussi, for alleged crimes against humanity committed while trying to put down last year's bloody revolt.
Jehani did not say what the documents were, except that they had been sent by Mohammed Ismail, Saif's former right-hand man, who has been on the run since the revolt.
Libya has been at loggerheads with the ICC over who has the right to bring accused former regime figures such as Saif and Senussi to justice.