"Although the Libyan government has danced around the issue, let's be very clear: if convicted (in Libya) Mr Gaddafi will be hanged," Melinda Taylor, a court-appointed lawyer, told judges amid a dispute between the ICC and Tripoli on where Seif should face justice.

While the ICC wants Saif, the only son of the slain Libyan leader in custody, to be tried in The Hague, Libya's post-revolutionary authorities insist he should stand trial in his home country.

Libya's lawyers told a three-judge bench on Tuesday the country had enough evidence to charge Saif with crimes against humanity, committed when Gaddafi and his loyalists tried to put down Libya's bloody revolution last year.

But the lawyers admitted that although Tripoli was committed to a fair trial for Gaddafi, it was a "complicated process and that Libya needed more time."

Taylor, a member of the ICC's defence office, said Gaddafi's right to a fair trial was being violated while he is held in isolation in the northwestern Libyan hilltown of Zintan, where he has been in custody since his arrest on November 19.

The ICC issued arrest warrants for Seif, 40, and Gaddafi's former spymaster Abdullah Senussi, 63, in June 2011 on two counts of crimes against humanity committed while trying to crush the uprising that ended over four decades of Gaddafi's iron-fisted rule.

But the ICC's jurisdiction is complementary to that of national courts, and it can only act when a member state is unwilling or unable to do so.

Libyan officials had asked in May for the court to quash a surrender request and throw out the case. The court's judges are to make a ruling at an unspecified later date.

Taylor on Tuesday cited a law passed by Libya's post-revolutionary National Transitional Council (NTC) which said no child of Gaddafi will ever benefit from leniency.

Putting Gaddafi on trial will "not be motivated by a desire for justice but a desire for revenge," Taylor told Wednesday's hearing, adding that both Saif and Senussi, extradited from Mauritania last month, were yet to appear before a judge.

"Mr Gaddafi is not a guinea pig (for Libyan justice). He is a person with rights. He should not be languishing in prison while Libya tries to build a judicial system," she said.

Asked to clarify dates for Saif and Senussi's possible trials, Philippe Sands who represents Tripoli in the case said: "The best estimate for a start date is February 2013."

A third warrant for Muammar Gaddafi was scrapped after he was killed by rebel forces on October 20 last year.

The UN estimates that up to 15,000 people were killed in the conflict, but Libya's NTC put the figure as high as 30,000.

The ICC is the world's only permanent criminal tribunal set up to try genocide, crimes against humanity and war crimes.