NATO chief Anders Fogh Rasmussen on Thursday expressed "strong regret" at the detention of an International Criminal Court delegation in Libya and urged their swift release.

"I strongly regret that certain groups in Libya have arrested or withheld representatives of the International Criminal Court and I would urge them to release those individuals as soon as possible," the Secretary General said in Australia.

Rasmussen noted that the "new authorities in Libya have co-operated in a positive manner" with the ICC, which he hoped augured well for the quick release of the four-person team, which includes Australian Melinda Taylor.

Taylor and her colleagues from Lebanon, Russia and Spain were detained last week after meeting with Seif al-Islam, the son of slain Libyan dictator Moamer Kadhafi, to help him choose a defence lawyer.

The Hague-based ICC wants to try Seif, 39, for crimes against humanity.

Libyan officials have alleged Taylor was carrying a pen camera and attempting to give Seif a coded letter from his former right-hand man Mohammed Ismail, who is on the run.

Australia has said that Taylor and the other three are entitled to immunity but warned Wednesday there appeared little chance of their early release, with indications that Libyan authorities may seek to hold them for 45 days.

On Tuesday, Libyan government spokesman Mohammed al-Harizi told the Australian Broadcasting Corporation that Taylor would be freed if she gave them information on Ismail.

"We want this guy. It is very important to catch this guy because this guy is very, very, very danger(ous) for us," he said, and claimed Taylor had met with Ismail, given that she had a letter from him.