Syrian authorities have killed thousands of detainees as part of a state policy of extermination, UN rights investigators said Monday in Geneva, accusing officials of crimes against humanity.

Islamic State extremists had also committed crimes against humanity by killing and torturing prisoners, the investigators of the Commission of Inquiry on Syria said in a new report.

In addition, anti-government rebels and al-Nusra Front, al-Qaeda's Syrian branch, were responsible for killing detainees to a lesser degree, according to the findings.

"Deaths in custody continue to occur in near-total secrecy and are largely lost to the international public," the report said.

Since the conflict started in March 2011, Syrian authorities had rounded up civilians across the country who were suspected of supporting the opposition or of being disloyal to the government of Bashar al-Assad.

Tens of thousands of people are being held by the government at any given time, said the panel, chaired by Brazilian legal expert Paulo Sergio Pinheiro.

The panel collected evidence that detainees are being executed or die of torture, hunger, or untreated diseases in prisons operated by various security authorities.

"Civilian superiors at the highest levels of government (...) have knowledge of the crimes or allegations of crimes being committed by their subordinates" or have consciously disregarded such information, the human rights investigators said.

They called on the UN Security Council to adopt sanctions against those responsible, and to launch proceedings at the International Criminal Court in The Hague.

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