Syrian activists and other citizens have vanished into secret detention as part of a "widespread campaign of terror against the civilian population" and a tactic of war by the Damascus government, UN investigators say.
The state-run practice of enforced disappearances in Syria - abductions that are officially denied - is systematic enough to amount to a crime against humanity, they said in a report.
"It is organised, it is a policy, it is widespread and consistent," Karen Koning AbuZayd, an American on the United Nations commission of inquiry, told Reuters.
"We heard from defectors that they were told if they arrested people in a demonstration or house to take all men between 18 and 40. So it is definitely directed at people they think may be fighters," she said, speaking from Chicago.
The UN team of 25 experts interviewed victims and relatives in Syria and abroad, documenting 100 cases to date, but believes the real number is likely to be in the thousands.
Some armed groups in northern Syria, especially al Qaeda's Islamic State of Iraq and the Levant (ISIL), have abducted people into incommunicado detention and denied their captivity, tantamount to enforced disappearances, the report said, citing two cases.
"These newish extremist groups are pretty good about concealing what they do. We don't yet have people coming out who have been their victims to tell us about it," AbuZayd said. Investigators hoped to gather more testimony for their next report in March.
In a separate report, London-based Amnesty International said ISIL was perpetrating "a shocking catalogue of abuses" in secret jails across northern Syria, including torture, flogging and killings after summary trials.
But the UN team said most witnesses identified soldiers, intelligence officers, and militias loyal to President Bashar al-Assad as being responsible for snatching people.
"In Syria, silence and fear shroud enforced disappearances. In several cases, individuals who reported a disappearance were themselves detained," said the report by the independent investigators led by Brazilian Paulo Pinheiro.
Despite the "organised nature" of the arrests and detentions, authorities fail to record the names and personal details of such detainees, including those who die, making it difficult to trace them and inform families, the report said.
Some bodies are returned bearing signs of severe torture.
The first victims were protesters in the revolt against Assad that began in March 2011. But as Syria descended into civil war, targeting spread to include people snatched at checkpoints and in their homes, it said.
Among those who disappeared were wounded patients in hospital suspected of links to rebels, doctors providing medical care in opposition areas, and families of defectors.
The mother and brother of a British surgeon who died in a Syrian prison days before his planned release this week are pleading with authorities to return the man's body and put an end to their family's 13-month ordeal.
"For me it is a really grievous crime. 'Should I continue to look, to hope? I don't know whether they are alive or dead.' That is what is really awful," AbuZayd said.
CRIME AGAINST HUMANITY
"There are reasonable grounds to believe that enforced disappearances were committed by government forces, as part of a widespread and systematic attack against the civilian population, and therefore amount to a crime against humanity," the report said.
The investigators, who include former UN war crimes prosecutor Carla del Ponte, said that there was no statute of limitations for such crimes.
"Investigating each case of enforced disappearance will remain the responsibility of the Syrian state regardless of the government in power," the report said.
With a view to future prosecution, the UN investigators have already drawn up two confidential lists of suspected war criminals on both sides, naming individuals as well as units.
More than 100,000 have been killed and millions forced from their homes in Syria's civil war.
Amnesty said that ISIL, one of the most powerful jihadi groups to emerge, operates seven clandestine prisons in rebel-held areas, dispensing torture and summary killings.
Detainees are held for reasons ranging from suspected theft to offences against Islam such as smoking or sex outside marriage. Others are seized simply for challenging ISIL authority or belonging to rival armed groups, it said.
"Those abducted and detained by ISIL include children as young as eight who are held together with adults in the same cruel and inhuman conditions," said Philip Luther, Amnesty's Director for the Middle East and North Africa.
Urging world powers to halt the flow of arms to ISIL and other armed groups accused of war crimes, Amnesty said Turkey in particular should prevent jihadi fighters and weapons crossing its border into northern Syria.
Gulf Arab states that back the anti-Assad rebels and are viewed as a main source of funding for the radical armed groups should also cut off flows of arms and equipment, Amnesty said.
The dominance of ISIL and other hardline rebel groups, which reject next month's planned Syria peace talks, has eclipsed more moderate, Western-backed rebels, fracturing the armed struggle against Assad and prompting Western alarm that al Qaeda is building a stronghold in northern Syria.