Syrian authorities are systematically detaining and torturing children, the United Nations' human rights chief, Navi Pillay, has told the BBC.
Ms Pillay said President Bashar al-Assad could end the detentions and stop the killing of civilians immediately, simply by issuing an order.
Syria has accepted a peace plan, amid scepticism about its intentions.
Most opposition groups have now agreed that the Syrian National Council will formally represent the Syrian people.
Navi Pillay, in an interview with the BBC before Syria accepted the plan, said Mr Assad would face justice for the abuses carried out by his security forces.
Asked if he bore command responsibility for the abuses, Ms Pillay said: "That is the legal situation. Factually there is enough evidence pointing to the fact that many of these acts are committed by the security forces [and] must have received the approval or the complicity at the highest level.