Ex-Gaddafi spy chief Abdallah Al-Senussi has been charged with illegally entering Mauritania.
The move will delay his extradition to face charges of murder and human rights abuses elsewhere.
Senussi was caught in Mauritania in possession of false identity documents in March.
Nicknamed the “butcher”, he is wanted in Libya for his alleged role in killing more than 1,200 prison inmates in Tripoli.
Known to be one of former Libyan leader Colonel Gaddafi’s closest aides, he also faces extradition charges from the International Criminal Court to answer for his role in the violent suppression of last year’s protests which eventually toppled Gaddafi’s regime.
Diplomatic sources say the US wishes to question him about the 1988 Lockerbie bombing over Scotland which killed 270 people.
He is also sought by the French in connection with a 1989 bombing over Niger which killed 54 French nationals. As yet no court date has been scheduled for the charges in Mauritania.