In a new report, the human rights experts identified more than 40 government-run detention centers with documented torture cases and said widespread attacks and sieges on civilian areas in Syria by pro-government forces are leading to mass casualties, malnutrition and starvation.
They said rebels have committed war crimes, including murder, executions, torture, hostage-taking, enforced disappearances, rape and using child soldiers.
But the commission chaired by Brazilian diplomat and scholar Paulo Sergio Pinheiro also concluded the five permanent UN Security Council members - Britain, China, France, Russia and the US - have failed to act on all of Syria's "grave violations" that threaten international peace and security.
"Such inaction has provided the space for the proliferation of actors in the Syrian Arab Republic, each pursuing its own agenda and contributing to the radicalisation and escalation of violence," their report said. "The Security Council bears this responsibility."
The panel, authorised by the UN's 47-nation Human Rights Council in Geneva, has previously concluded that it believes the Syrian government is committing a crime against humanity by making people systematically vanish, and that rebels have also been making their opponents disappear and running secret prisons.