Four International Criminal Court staff have been arrested in Libya during a visit to arrange the defence for slain dictator Moamer Kadhafi's detained son Seif al-Islam, the tribunal said Saturday.
A commander with the Zintan brigade, the former rebel fighters who captured Seif and are still holding him, said one of the lawyers with the group had been arrested after trying to pass "dangerous" documents to him.
An ICC statement called for their release and urged Tripoli to ensure that they were safe. Their team had been detained since Thursday, it added.
Earlier Saturday, sources in Libya said Australian lawyer Melinda Taylor had been arrested after trying to pass documents to Seif.
Ahmed al-Jehani, Libya's representative to the ICC, said Taylor was "under house arrest in Zintan, not in prison" and was being questioned. Zintan lies about 180 kilometres (110 miles) southwest of the Libyan capital.
A Libyan foreign ministry official said they would asking the ICC to lift the immunity that goes with Taylor's status as an ICC official so they could proceed with their investigation.
Ajmi al-Atiri, commander of the Zintan brigade, said a team of four ICC delegates had come to visit Seif al-Islam on Thursday and had asked to meet him in private, a request they had been refused.
Two female ICC delegates were searched by a policewoman after the visit and the documents were found on the ICC lawyer.
They had also found a camera on the translator with the ICC team. Atiri accused her of having falsely portrayed herself as a nurse and of having carried a camera with her into the meeting.
They had no intention of releasing Taylor, stressing that since she had been detained in Zintan, investigations would be conducted there, he added.
ICC president Judge Sang-Hyun Song said in the tribunal's statement from The Hague: "We are very concerned about the safety of our staff in the absence of any contact with them.
"These four international civil servants have immunity when on an official ICC mission.
"I call on the Libyan authorities to immediately take all necessary measures to ensure their safety and security and to liberate them."
Other members of the ICC team were officials handling administrative aspects of the case, said the ICC statement. They were there to help Seif choose a defence lawyer.
Taylor works with Xavier-Jean Keita, the defence attorney appointed by the ICC. Contacted by AFP, Keita declined to make any comment.
Although the ICC delegation's visit was authorised by Libya's chief prosecutor, the complaint was that Taylor had tried to deliver the documents without first declaring them.
The documents in question included an unsigned letter from Seif to the ICC saying "there is no government or law in Libya" and that he was being "ill-treated", he said. There was also a blank document signed by him.
Atiri also said the brigade had been "surprised" that National Transitional Council chief Mustafa Abdel Jalil had made telephone calls requesting the "immediate release" of the detainees.
But a Libyan foreign ministry official said it would ask the ICC to waive the lawyer's immunities so that formal investigations could begin.
"I think the woman will be with us for a while until the waiver is granted by the International Criminal Court so we can formally start the investigations," Mohammed Abdulaziz told AFP.
The delegation should have formally declared the documents, he said.
"It is an act that is jeopardising the national security of Libya and we are taking it very seriously," he added.
The ICC wants to try both Seif, 39, and his late father's spymaster, Abdullah Senussi, for crimes against humanity committed while trying to put down last year's bloody revolt.
But the new regime in Libya wants to put Seif on trial in a local court.
The ex-rebels in Zintan who have been holding Seif since they captured him on November 19 have refused to transfer him to Tripoli until they are given months of back pay.
Kadhafi himself was captured and killed by rebel forces on October 20.