Libya has been building up evidence against Seif al-Islam Gaddafi in the hope of persuading International Criminal Court judges to sanction a local trial, an ICC envoy said on Saturday.

"The general prosecutor explained to us what he was doing and he has a very good case," Luis Moreno-Ocampo told journalists in Tripoli at the close of a trip to follow up on Seif's case and push forward investigations on rape.

"I understand there are more than 30 witnesses," he said.

"He has witnesses, interceptions, documents; apparently he has a lot of evidence," Ocampo told AFP, adding he had not seen the body of proof because it remains confidential at this stage of the investigations.

Libya has been at loggerheads with the ICC over who has the right to bring former regime figures such as Seif-al Islam and the dead dictator's spymaster, Abdullah Senussi, to justice.

"Libya on April 30 will present its arguments on why Libya wants to do the trial of Seif Gaddafi and Senussi," 62, who was detained in March in Mauritania, Ocampo said.

There is no deadline for judges to make a decision, but Ocampo said that, based on his estimates, it seems unlikely Libya will be able to uphold its vow to try Seif ahead of elections for a constituent assembly in June.

Gaddafi's son remains in the custody of a brigade in the town of Zintan which has been reticent to hand him over to the new authorities. Ocampo said Libyan officials told him they were confident he is "not mistreated."

"Zintan authorities have to show that his rights are respected," he said.

Ocampo said it was critical for Libyans who fought against the injustices of Moamer Gaddafi's regime last year to show that they can now "respect justice for a person like Seif."

The prosecutor said his mission had also focused on how to tackle the taboo topic of rape in the hope of collecting evidence against top officials of the former regime who are now outside the country.

"It is very important for victims that the leaders talks about their suffering. Libya has to give recognition to these people, otherwise it will be double punishment: they were raped and now they are marginalised."

Ocampo said last June that ICC investigators had evidence that Gaddafi had ordered his fighters to commit rape and supplied them with sexual stimulants like Viagra.