UN human rights Chief Navi Pillay has made an appeal for "further militarization" of the conflict in Syria to be avoided "at all costs".
She said both government forces and the opposition have been "involved in actions harming civilians".
She also renewed her call for the UN Security Council to refer the conflict to the International Criminal Court.
Pillay did not mention any countries by name in her remarks, but Saudi Arabia and Qatar are widely believed to be supplying arms to the rebel Free Syrian Army and have been paying their salaries.
Russia and Iran have been accused of arming Syrian government forces.
Pillay said government forces had attacked clinics, and that rebels had taken over at least one medical facility for military purposes.
She also said there was a report of anti-government forces using children as human shields.
She also urged that the mandate of the UN observer mission in Syria be strengthened so it could effectively report on the human rights situation.
The Security Council is considering whether to downscale the mission, since the violence has made it impossible for it to operate.