Interior Minister Matteo Piantedosi defended before parliament the release on Tuesday of Osama Najim, who is wanted by the International Criminal Court (ICC) for war crimes and crimes against humanity charges related to his management of migrant detention camps.

Najim was arrested in the northern city of Turin on Sunday but returned to Tripoli Tuesday on an Italian air force plane after the court of appeals in Rome ruled that he could not be held.

Piantedosi told the Senate that the court had found the detention of Najim was "irregular" and "not provided for by law", ordering him freed.

Najim was "then repatriated to Tripoli for urgent security reasons", the minister said, citing "the dangerousness of the subject".

Najim 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 2015.

Italy's release of the Libyan has drawn vehement criticism from opposition parties and a subtle rebuke from the ICC, which on Wednesday reminded its member state that it had a "duty" to "cooperate fully" in the court's investigations and prosecutions.

It said Najim had been released and sent home "without prior notice or consultation with the court".

In its order Tuesday to release Najim, the Rome appeals court wrote that the arrest did not conform to Italian law because ICC requests should first pass through the justice minister, who, "to date, has sent no request on the matter".

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Italian opposition parties have demanded that Prime Minister Giorgia Meloni explain before parliament the reasons for the release of an accused war criminal wanted by the international court.

On Thursday, Sandra Zampa, a senator with the centre-left Democratic party, called the affair "shameful".

"He was not simply released from prison, but he was brought home on a state plane," Zampa said, charging that "procedural errors have nothing to do with it".

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

Rome has a controversial deal with the North African country -- dating from 2017 and renewed under Meloni's hard-right government -- to provide 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.

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

Najim's arrest and release come about a week after Rome and Tripoli resumed direct flights between the two capitals after a decade-long hiatus.

Italy's foreign ministry hailed the "concerted effort" shown by Rome to strengthen ties with its former colony, calling Libya "a strategic and privileged partner for our country".