Muammar Gaddafi's former spy chief Abdullah al-Senussi will stand trial in Mauritania for illegally entering the country, judicial sources said on Monday.
The move would delay efforts to have him face international justice.
Senussi's arrest in the West African state in March started a three-way tug-of-war among Libya, France and the International Criminal Court (ICC) for the extradition of one of the most feared members of Gaddafi's fallen regime.
"Abdullah al-Senussi has been interviewed for the first time by the state prosecutor. He should face trial soon for illegal entry into Mauritanian territory," said one judicial source, who requested anonymity. A second judicial source, though, confirmed that Senussi had been formally charged.
The decision to proceed with the charges comes after the expiry of Senussi's 45-day custody period and could mean any extradition of Senussi to face justice outside Mauritania is indefinitely delayed, especially if he is found guilty.
In Libya, Saad al-Shelmani, the foreign mnistry spokesperson, said: "We hope that this is the beginning of the process to hand over Senussi to Libya where he will face a trial by his own people."
Senussi was arrested as he flew in to the Mauritanian capital, Nouakchott from Morocco on a falsified Malian passport.
He is suspected of playing a central role in the killing of more than 1,200 inmates at Tripoli's Abu Salim prison in 1996.
It was the arrest of a lawyer acting for relatives of the victims that sparked the revolt in February last year that led to the ousting of Gaddafi by NATO-backed rebel forces.
France wants Senussi in connection with a 1989 airliner bombing over Niger in which 54 of its nationals died.
Senussi has also been linked to the 1988 bombing over Lockerbie, Scotland of a PanAm jet that killed 270 people.
Diplomatic sources have said the United States was keen to question him about that attack.
Mauritania revealed last month that Senussi was ill, suffering from unspecified health problems that were already present when he arrived in Nouakchott.
Libya's new government sent a top-level delegation to Mauritania in the days after his arrest and announced that it had secured a deal in principle for his extradition – an assertion immediately played down by Mauritanian officials.
The former French colony has traditionally strong ties to France, which made clear its desire to try Senussi under the presidency of Nicolas Sarkozy - who has since been replaced by his Socialist challenger Francois Hollande.