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, UN investigators 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," an independent commission looking into the issue said in a report on Thursday.
Most witnesses have identified Syrian intelligence officers, soldiers and pro-government militias as having snatched people whose fate remains unknown, according to the independent investigators led by Brazilian Paulo Pinheiro.
The commission also said that some armed groups in northern Syria, especially the Islamic State of Iraq and the Levant (ISIL), have begun to abduct people into incommunicado detention and deny their captivity. ISIL has also sought ransoms or prisoner exchanges, which constitute separate war crimes, it added.
In a separate report, London-based Amnesty International said armed groups were perpetrating "a shocking catalogue of abuses" in secret jails across northern Syria, including torture, flogging and killings after summary trials.
Seized for smoking
It said that ISIL, one of the most powerful rebel groups to emerge from Syria's almost three-year-old conflict, is operating seven clandestine prisons in rebel-held areas.
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.
Human rights abuses have been rife in Syria's civil war, with human right groups and the UN repeatedly warning about the worsening practices in the country.
ISIL, along with some other radical groups, rejects next month's planned peace conference in Switzerland city of Geneva that is due to bring Assad's government and his opponents to the negotiating table for the first time since the conflict erupted in 2011.