* Enforced, systematic abductions amount to war crimes
* Bodies returned bearing signs of torture
* Amnesty reports "shocking abuses" by radical fighters
By Stephanie Nebehay
GENEVA, Dec 19 (Reuters) - Syrian activists and other citizens have vanished into secret detention as part of a "widespread campaign of terror against the civilian population" by the Damascus government, U.N. investigators said on Thursday.
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.
Some armed groups in northern Syria, especially the Islamic State of Iraq and the Levant (ISIL), have also abducted people into incommunicado detention and denied their captivity, tantamount to the crime of enforced disappearances, it said.
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 United Nations investigators said most witnesses identified Syrian intelligence officers, soldiers and militias loyal to President Bashar al-Assad as having snatched people whose fate remains unknown.
"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, it 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.
Wounded patients in hospital suspected of links to rebels, doctors providing medical care in opposition areas and families of defectors have disappeared, the investigators said, quoting first-hand accounts, mainly from Syrians who have fled abroad.
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.
THOUSANDS OF CASES
More than 100 cases of enforced disappearances have been documented, but the total number of cases is likely to be in the thousands, one U.N. official said.
"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 U.N. 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 U.N. investigators have already drawn up two confidential lists of suspected war criminals on both sides, naming individuals as well as units believed to have carried out atrocities.
More than 100,000 people have been killed in the conflict and millions forced from their homes, according to U.N. figures. Shi'ite fighters from Iraq and Lebanon's Hezbollah have fought alongside Assad's army, while Sunni militants have flocked from across the Islamic world to join the rebels.
Amnesty said that ISIL, one of the most powerful jihadi groups to emerge from the almost three-year-old conflict, 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 and other abuses, 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.
Amnesty said several children had received severe floggings and on one occasion a father had to listen to his son's screams in a nearby room. Two detainees said they witnessed a child of about 14 receive 90 lashes at prison in Raqqa province.
"After years in which they were prey to the brutality of the Assad regime, the people of Raqqa and Aleppo are now suffering under a new form of tyranny imposed on them by ISIL, in which arbitrary detention, torture and executions have become the order of the day," Luther said. (Additional reporting by Dominic Evans; Editing by Alistair Lyon)