The trial of Saif al-Islam Gaddafi, a son of the ousted Libyan leader sentenced to death in absentia, did not meet international standards and he should face murder charges at the International Criminal Court (ICC), the United Nations said on Tuesday.

Since his father's toppling in 2011, Saif has been held in Zintan, a mountainous western region, by one of the factions that began contending for power after Gaddafi was killed.

He was sentenced to death in July 2015 by a Tripoli court for war crimes, including killing protesters during the revolution. Zintani forces refused to hand him over, saying they did not trust Tripoli to guarantee he does not escape.

The U.N. report on the trial of 37 defendants including Saif cited serious violations of due process, such as prolonged incommunicado detention without access to families or lawyers, and allegations of torture that were not properly investigated.

The proceedings "fell short of international norms and standards for fair trial and also breached Libyan law in some respects", it said. No prosecution witnesses were called to testify in court, undermining the defendants' ability to challenge evidence.

"This trial was a missed opportunity for justice," U.N. High Commissioner for Human Rights Zeid Ra'ad Al Hussein said in a statement.

Saif, former intelligence chief Abdullah al-Senussi and former Prime Minister Al-Baghdadi a-Mahmoudi were among nine defendants sentenced to death by firing squad.

"The Libyan Government has been unable to secure the arrest and surrender of (Gaddafi), who remains in Zintan and is considered to be outside the control of the internationally-recognised Libyan authorities," the U.N. report said.

Libya's Court of Cassation is to review the procedures in the case but not the facts and evidence. "As such the review does not constitute a full appeal as required by international standards," the U.N. report said.

It called on Libyan authorities to ensure the surrender of Saif to the Hague-based ICC, "in compliance with Libya’s international obligations". The ICC does not allow the death penalty.

"The (ICC) Prosecutor continues to seek the surrender of (Gaddafi) to the Libyan authorities in order to proceed with his case," it said.

The report, by the U.N. human rights office and U.N. Support Mission in Libya (UNSMIL), urged Libyan authorities to reform the criminal justice system saying that the trial had highlighted "major flaws".