The expert panel said it found "a consistent country-wide pattern" of Syrian security, armed forces and pro-government militia seizing people from mass arrests, house searches, checkpoints and hospitals, then making them disappear.
Among the cases were a 60-year-old woman put in Homs prison for asking about her missing son, and a peaceful protester's brother taken in a house raid by political security agents.
An Air Force defector has described orders not to provide information about the whereabouts of detainees or to speak to their relatives, the report said.
The four experts reported yesterday that rebel groups such as the al Qaida-linked Islamic State in Iraq and the Levant that control large parts of northern Syria, had also begun seizing people and running secret prisons.
They said the opposition's abductions of human rights advocates, journalists, activists, humanitarian workers, religious leaders and perceived supporters of President Bashar Assad's government differed because victims were hostages.