Australia has called for the release of lawyer Melinda Taylor, who was held after trying to meet Saif al-Islam, son of the late leader Muammar Gaddafi.
Prime Minister Julia Gillard said Libya should "expedite the end" of the Australian defence lawyer's detention.
She is one of four International Criminal Court's (ICC) staff detained in Zintan town last week.
Reports say local people became suspicious of documents she tried to pass to Saif al-Islam.
"I am very concerned about the detention of Ms Taylor," Ms Gillard told reporters. "We are calling on the Libyan government to expedite the end of Ms Taylor's detention."
Australia's Foreign Affairs Minister Bob Carr said he raised Ms Taylor's case with Libyan Deputy Foreign Minister Muhammad Aziz.
He said that Mr Aziz confirmed in a phone call that Ms Taylor was being detained in Zintan pending further inquiry.
"An ICC team is in Libya to engage directly with authorities on the matter," he added.
Ms Taylor's parents released a statement, saying that they did not wish to publicly comment on the situation and asked media to respect their privacy.
"Our thoughts, at this time, are very much with our daughter, Melinda, and her colleagues," they said.
The ICC delegation had travelled to Libya on 6 June to meet Saif al-Islam Gaddafi in Zintan.
Members of the brigade holding Saif al-Islam say the ICC team were carrying documents, including a letter from a former confidante of his who is now in Egypt, about 135km (85 miles) south-west of Tripoli.
Saif al-Islam, who was captured last November by militiamen as he tried to flee the country, has been indicted by the ICC for crimes against humanity.
Libya's interim government has so far refused to hand him over for trial in the Netherlands - the seat of the ICC. Libya has insisted he should be tried by a Libyan court.