The top U.N. human rights official called on Tuesday for tens of thousands of detainees to be released from Syria's prisons and for torturers to be brought to court as part of a lasting peace.
Former Syrian detainees also testified before the U.N. Human Rights Council about their suffering and concern for men, women and children still in custody of the government or of extremist groups including al-Nusra and Islamic State.
"Today in a sense the entire country has become a torture-chamber; a place of savage horror and absolute injustice," said U.N. High Commissioner for Human Rights Zeid Ra'ad al-Hussein.
"Ensuring accountability, establishing the truth and providing reparations must happen if the Syrian people are ever to find reconciliation and peace," he told the Geneva forum.
Zeid urged the warring sides to halt torture and executions and to free detainees or at least provide basic information to their families.
The Syrian government delegation did not attend but has denied allegations of systematic torture. The envoy from Russia, its main ally, called the event a "waste of valuable time".
Noura Al-Ameer, a former detainee and activist, cited the case of Ranya, a woman detained in 2012 with six of her children and still missing.
"Many other women are detained with their children, detained in places not even fit for animals, let alone fit for children," al-Ameer told the council.
Paulo Pinheiro, chairman of the U.N. Commission of Inquiry, noted that its 2014 report found that the scale of deaths in prisons indicated that the Assad government was responsible for "extermination as a crime against humanity".
"Too many voices have been silenced by enforced disappearance, arbitrary detention, or death," he said.
Zeid and Pinheiro pledged support to a new U.N. mechanism which will collect evidence and prepare criminal files for prosecution by national authorities or an international court.
The government detains 87 percent of those in custody, said Fadel Abdul Ghani, executive director of the Syrian Network for Human Rights.
"The regime is trumping everyone else with nearly 92,000 individuals that are still inside its detention centers," said Abdul Ghani. Many suffer "horrendous acts of torture", he said.
Mazen Darwish, a lawyer freed in 2015 after three years in jail, voiced outrage at the lack of international action.
"We are speaking of a daily massacre going on for six years. Why are we here? Today there are women, men, children, innocent people who are being killed under torture...It is strange that in front of all this evidence we do not see a real movement."
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