The Hague-based court said in a statement that Italian authorities had arrested the suspect in Turin on Sunday, but reportedly released him Tuesday "without prior notice or consultation with the Court".

"On 18 January 2025, Pre-Trial Chamber I of the International Criminal Court by majority, issued a warrant of arrest for Mr Osama Almasri Najim," the ICC said.

Najim, also referred to as Osama Elmasry Njeem, is believed to have been in charge of Tripoli's Mitiga detention centre, and is wanted on charges including murder, rape and sexual violence and torture, committed since February 15, 2015.

The alleged crimes were committed against detainees because of their religion, perceived "immoral behaviour" or alleged support or affiliation to other armed groups, the ICC said.

Najim was arrested Sunday at a Turin hotel on the ICC warrant after a tip from Interpol, Italian media reported.

But on Tuesday "without prior notice or consultation with the Court, Mr Osama Najim was reportedly released from custody and transported back to Libya," the ICC said.

"The Court is seeking, and is yet to obtain, verification from the authorities on the steps reportedly taken," it said.

The ICC, to which Rome is a state party and whose founding statute bears the Italian capital's name, said it had "deliberately" refrained from commenting on Najim's arrest "at the request of, and acting out of full respect for the Italian authorities".

"At the same time, the Court continued to pursue its engagement with the Italian authorities to ensure the effective execution of all steps required by the Rome Statute for the implementation of the Court's request."

But on Tuesday, Najim was released and returned to Libya without explanation.

- 'Cooperate fully' -

The ICC is an independent court capable of prosecuting those accused of the world's worst crimes.

Its 125-member states, including Italy, which ratified the Rome Statute in 1999, are legally required to carry out arrest warrants issued by the court's judges.

"The Court recalls the duty of all States Parties to cooperate fully with the Court in its investigations and prosecutions of crimes," the ICC said.

Italian opposition parties have demanded Prime Minister Giorgia Meloni come to parliament to explain the decision to release Najim, with Democratic Party leader Elly Schlein calling it "very opaque".

"Giorgia Meloni wanted to chase human traffickers around the world, a Libyan one was arrested in Italy and instead of following up the request of the ICC... they sent him back unpunished to Libya," the centre-left opposition leader said.

Maria Elena Boschi, a lawmaker from the centrist Italia Viva party, said it was "astonishing that a person with an arrest warrant from the ICC feels so safe in our country that he can be a tourist in Turin and even go to see a match at the stadium."

"After being arrested, he is released through a legal technicality and then even taken back to Libya on a flight from our air force," she said.

International human rights groups have long condemned abuses in Libyan detention centres, citing widespread violence and torture.

Rome's controversial deal with Libya -- dating from 2017 and renewed under Meloni's hard-right government, involved providing funding and training to the Libyan coastguard.

In exchange, Libya was expected to help stem the departure of migrants to Italy, or return those already at sea back to Libya, where they were often taken to such detention centres.

The situation in Libya was first referred to the ICC for investigation by the United Nations in early 2011, a few months before a revolt toppled dictator Moamer Kadhafi after four decades of iron-fisted rule.

Seven arrest warrants are still pending in the Libya case, the ICC said.

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