Long the heir, Seif was also referred to as his father's "de facto prime minister" The Hague  - AFP The International Criminal Court said it was in contact with slain Libyan leader Muammar Gaddafi's son Seif al-Islam over his surrender, as NATO decided to end its mission in Libya .
Luis Moreno-Ocampo warned however that the ICC head learned that a group of mercenaries had offered to move Seif to an African country that was not party to the ICC's founding document, the Rome Statute.
"Through intermediaries, we have informal contact with Seif," the prosecutor said in a statement issued at the court's headquarters in The Hague.
"The Office of the Prosecutor has made it clear that if he surrenders to the ICC, he has the right to be heard in court, he is innocent until proven guilty," Moreno-Ocampo said. "The judges will decide."
After learning that a group of mercenaries had offered to move Seif to an African state not bound to hand him over to the ICC, his office was "exploring the possibility" of intercepting any plane carrying him to make an arrest.
Seif, 39, and Gaddafi's security chief and brother-in-law Abdullah al-Senussi, 62, are the most wanted fugitives from the slain despot's inner circle.
They are wanted by the ICC on charges of crimes against humanity, committed after the start of the uprising against Gaddafi's regime in mid-February. The ICC issued arrest warrants against Gaddafi, Seif and Senussi on June 27.
Interpol issued "red notices" for their arrest on September 9.
Following Gaddafi's death, Interpol Secretary General Ronald Noble and Moreno-Ocampo issued a joint statement last week urging Seif to turn himself in, promising him safe passage to The Hague.
But in Tripoli, interim justice and human rights minister Mohammed al-Allagi of Libya's National Transition Council (NTC) said Seif would be tried in Libya if he were caught there.
"If he were caught in Libya, Libyan law states that he would have to be tried here. But he would be entitled to a fair trial," Allagi said in response to a journalist's question.
But the international justice director at New York-based Human Rights Watch said a fair trial in Libya could not be guaranteed.
A Libyan trial "on these complex crimes in a highly charged environment is not likely to render justice in an independent way", Richard Dicker told AFP.
Gaddafi's "gruesome" death last week had highlighted Libya's volatility, he said.
"Seif al-Islam's appearance before the ICC will best ensure that justice is done," he added.
Whether or not Seif decided to surrender, he and Senussi must be allowed to do so safely and their rights "guaranteed", rights group Amnesty International urged.
"The National Transitional Council and neighbouring governments must ensure their safe detention and prompt transfer to The Hague for investigation, whether they surrender voluntarily or are arrested and transferred," spokesman Marek Marczynski said in a statement.
Long the heir to Gaddafi's regime, Seif was also referred to as his father's "de facto prime minister", controlling the regime's finances and logistics, while Senussi controlled its security organs.
The exact whereabouts of the two remain unknown. But security sources from both Niger and Mali said Thursday that Senussi had crossed from Niger into Mali, with sources claiming he was under Tuareg protection.
NATO meanwhile said it would end its mission in Libya on October 31, declaring it had fulfilled its mandate to protect civilians. It called on the new regime to build a democratic Libya.
"We have fully complied with the historic mandate of the United Nations to protect the people of Libya, to enforce the no-fly zone and the arms embargo," NATO Secretary General Anders Fogh Rasmussen said in a statement.
"Operation Unified Protector is one of the most successful in NATO history," he said after NATO ambassadors formally agreed to end it.
Alliance warplanes will wind up the mission on Monday after flying more than 26,000 sorties and bombing almost 6,000 targets in a seven-month operation that helped a ragtag rebel force oust Gaddafi.
The conflict ended in controversial fashion when Gaddafi was shot dead on October 20, a killing that was criticised even by Western allies of the interim NTC regime.
Facing global criticism over Gaddafi's death, the NTC vowed on Thursday to bring his killers to justice, a sharp break with its previous insistence he was caught in the crossfire with his own loyalists.
While NATO has denied targeting Gaddafi during the campaign, it was an alliance air strike that hit his convoy as it fled Sirte, leading to his capture and killing.