GENEVA, Switzerland - A panel of U.N. investigators said Thursday it believes the Syrian government is committing a crime against humanity by making people systematically vanish, and that rebels have also recently begun making their opponents disappear.

In a report based on interviews with survivors and family members of victims, the panel said the war tactic being used by President Bashar al-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-Qaeda-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.

But until recently, it said, most of the opposition has been committing war crimes, a lesser category of violations, by abducting human-rights advocates, journalists, activists, humanitarian workers, religious leaders and perceived supporters of Assad's government.

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 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."

In the case of the Syrian government, the panel chaired by Brazilian diplomat and scholar Paulo Sergio Pinheiro said it found "a consistent country-wide pattern" of Syrian security, armed forces and pro-government militia seizing people in mass arrests or house searches and at checkpoints and hospitals, then making them disappear - and denying that they even exist.

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 said. "We know there are thousands, but we have no idea how many are in detention."

The U.N. panel pressed Syria's government to provide information and called on both sides to stop the practice, and said it could "only hint at the scope of the crisis . . . and the state of fear in which ordinary citizens live."