United Nations human rights investigators say war crimes have been committed in Syria by both government forces and opposition groups.
Syrian government forces and allied Shabbiha militia have committed war crimes including murder and torture of civilians in what appears to be state-directed policy, the investigators said.
Rebels fighting to topple President Bashar al-Assad had also committed war crimes, including executions, but on a smaller scale than those by the army and security forces.
The investigators' report called for the UN Security Council to take appropriate action given the gravity of documented violations by all sides in a 17 month conflict that investigators said had become a civil war.
Paulo Pinheiro, the commissioner who led the probe, said Syria's army of 300,000 had targeted rebel-held areas of cities with heavy artillery and helicopters.
It had "much more means to inflict war crimes, for example bombing civilian populations".
The Security Council can refer a case to the prosecutor of the International Criminal Court (ICC), the UN war crimes tribunal, but Russia and China - which have veto power - have been loath to condemn Syria, Reuters reports.
The independent investigators conducted more than 1,000 interviews over the past year, mainly with Syrian refugees or defectors who have fled to neighbouring countries, to produce their latest 102-page report to the UN Human Rights Council.