In a report based on interviews with survivors and family members of victims, the panel said the war tactic being used by President Bashar Assad's government amounts to a crime against humanity because it is part of a policy of spreading terror and mental anguish among those left wondering about their loved ones.
Rebel groups such as the al-Qaida-linked Islamic State in Iraq and the Levant (ISIL) that control large parts of northern Syria also have begun seizing people and running secret prisons, the panel said.
Those are war crimes because they are not systematic, the victims tend to be taken as hostages for ransom or prisoner exchanges, and their existence isn't concealed, the panel said.
Syria's main opposition group in exile, the Western-backed Syrian National Coalition, said any opposition groups illegally seizing and holding innocent civilians are violating of the basic goals of the Syrian uprising.
It said in a statement that the coalition does not consider ISIL a part of the opposition, "Its actions serve the regime's interests."
Most of the victims have been young men, and panel member Karen Koning AbuZayd said that based on "consistent reports" of how and where people are disappearing, the panel assumes "most of them" have been tortured in prison.
"It appears to be a policy because it's so widespread. It's around the country," she told The Associated Press. "We know there are thousands, but we have no idea how many are in detention.
