The International Criminal Court (ICC) demanded answers from Italy on Wednesday over why it freed a Libyan man suspected of crimes against humanity and war crimes, including murder, torture, rape and sexual violence.

Italy detained Osama Elmasry Njeem, also known as Osama Almasri Njeem, on Sunday in Turin after receiving a tipoff from The Hague-based ICC, which had issued an international arrest warrant against him.

However, the Italian government unexpectedly freed him on Tuesday because of a legal technicality, an Interior Ministry source said. He was immediately flown on an official state aircraft to Tripoli, where he received a hero's welcome.

The ICC, which has been investigating alleged serious crimes committed in Libya since the country's 2011 civil war, said in a statement it had urged Italy to contact its staff if there were any problems with the arrest process.

However, it said Njeem had been let go without prior notice or consultation.

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

Neither Prime Minister Giorgia Meloni's office nor the Justice Ministry has commented on the case, which is likely to cause the government deep embarrassment. The Interior Ministry source said Njeem was freed because local police had not immediately informed the Justice Ministry as required.

Njeem's release was a blow for the ICC, which is facing ferocious pushback in the United States after issuing arrest warrants for Israeli Prime Minister Benjamin Netanyahu and his former defence minister over Israel's campaign in Gaza.

"The Court recalls the duty of all (member states) to cooperate fully with the Court in its investigations and prosecutions of crimes," the ICC statement said.

OPPOSITION ANGER

Video on social media showed Njeem, who is a brigadier general in Libya's Judicial Police, being carried on the shoulders of supporters after he arrived at Mitiga Airport in Tripoli on Tuesday night.

Meloni's conservative administration depends heavily on Libyan security forces for preventing would-be migrants from leaving the North African nation and heading to southern Italy.

The ICC said the list of crimes listed in the arrest warrant had been "committed by Mr. Njeem personally, ordered by him, or with his assistance, by members of the Special Deterrence Forces".

It said the alleged crimes took place at Mitiga prison, against people who had been imprisoned for their religious beliefs, such as being Christian, for perceived contraventions of religious ideology, such as homosexuality, or for their suspected support of other armed groups.

Opposition politicians in Italy demanded an explanation from the government.

"Am I the only one who thinks you've lost your minds, or is this the image of a hypocritical and indecent government?" former Prime Minister Matteo Renzi told parliament on Wednesday.

Njeem's release came little over a week after Rome had abruptly released an Iranian businessman who had been detained on a U.S. warrant, in an apparent swap for an Italian journalist who had been arrested in Tehran.