Libyan Deputy Prime Minister Mustafa Abu Shagour arrived in Mauritania on Monday to press for the handover of Muammar Gaddafi's feared intelligence chief Abdallah al-Senussi, officials said.
Senussi, 62, the last major Gaddafi associate on the run since the dictator's overthrow and death in a popular revolt last year, was arrested in Mauritania after his arrival late on Friday on a flight from Morocco. France and the International Criminal Court (ICC) in The Hague also seek custody of him.
Mauritania has made no official statement on Senussi, accused of playing a central role in repression and torture under Gaddafi, apart from a brief report by its official state news agency. A security source speaking to Reuters on condition of anonymity said it had yet to take a decision on his fate.
Senussi was being held at the headquarters of Mauritania's security service in Nouakchott, sources there said. Diplomatic sources said he was carrying several false passports when detained.
Libyan Foreign Ministry spokesman Saad Elshlmani said it was not clear how long Shagour and his delegation would stay in Mauritania.
Asked by reporters at the airport why Libya wanted Senussi extradited, Shagour said: "Because he has committed crimes there." He declined to comment further.
The ICC has indicted Senussi for crimes against humanity, but Libya also wants him to face local justice. While Mauritania is not a signatory to the ICC, the court says it is bound to cooperate with it by a UN Security Council resolution.
A spokesman for the ICC said the tribunal was still waiting for a response from Mauritanian authorities after serving a warrant of arrest for Senussi.
France, 54 of whose nationals were killed in the bombing of a UTA airliner over Niger in 1989, is also seeking Senussi, who was sentenced in absentia to life imprisonment for his alleged role in the attack.
Senussi was Gaddafi's right-hand man and security experts said Mauritania's government would face massive diplomatic pressure in the three-way tussle over a man with access to some of the best-kept secrets of the Gaddafi regime.
"He is a very big fish, and he has a Pandora's box inside his brain. He knows everything about Gaddafi's rule - security and intelligence systems going back 30 years or more," said Khaeri Aboshagor, senior representative of the Libyan League for Human Rights.
Aboshagor, who is normally UK-based but was speaking from Libya, said there was "pretty strong" information suggesting Morocco allowed Senussi to leave for Mauritania. He did not elaborate and Morocco has not commented on the case.
Senussi's name has been linked to the 1988 bombing over Lockerbie, Scotland of a Pan Am jet that killed 270 people. He is also widely thought to know details of Gaddafi's cooperation with Western states.
Reed Brody, Brussels-based counsel for Human Rights Watch, said it was hard to discern any clear legal primacy over the competing claims for Senussi, paving the way for a bout of political and diplomatic manoeuvring over his fate.
While Mauritania is not a signatory to the ICC, a UN Security Council resolution urges all states to cooperate with the Hague-based court. Yet Brody said Nouakchott was unlikely to face any legal repercussions if it did not.
"It's a legal free-for-all. What's good is that everyone wants to prosecute him," Brody said by telephone.