Syrian authorities are deliberately and systematically targeting children, the United Nations' human rights chief, Navi Pillay, has told the BBC.
She said she was deeply concerned about the fate of hundreds of children being held in detention.
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.
The peace plan was put forward by UN and Arab League envoy Kofi Annan.
But US Secretary of State Hillary Clinton said President Assad would be judged by events.
"Given Assad's history of over-promising and under-delivering, that commitment must now be matched by immediate actions," she said.
"If he is ready to bring this dark chapter in Syria's history to a close he can prove it by immediately ordering regime forces to stop firing and begin withdrawing from populated areas," she added.
Navi Pillay told the BBC that the Syrian leader would face justice for the abuses carried out by his security forces.
Asked if President Assad bore command responsibility for the abuses, she said: "That is the legal situation. Factually there's 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.
"Because President Assad could simply issue an order to stop the killings and the killings would stop."
Ms Pillay said she believed that the UN Security Council had enough reliable information to warrant referring Syria to the International Criminal Court (ICC).
"I feel that investigation and prosecution is a crucial element to deter and call a stop to these violations," she said.
She listed what she called "horrendous" treatment of children during the unrest.
"They've gone for the children - for whatever purposes - in large numbers. Hundreds detained and tortured... it's just horrendous.
"Children shot in the knees, held together with adults in really inhumane conditions, denied medical treatment for their injuries, either held as hostages or as sources of information."
Ms Pillay said anyone who committed such violations would be held to account.
"There is no statute of limitations so people like [Mr Assad] can go on for a very long time but one day they will have to face justice."
Late on Tuesday, several Syrian dissident groups agreed to recognise the Syrian National Council as the official representative of the Syrian people, after a meeting in Istanbul aimed at unifying the opposition movement.
The BBC correspondent at the gathering says he has not met a single delegate who thinks the president is even remotely sincere, and Mr Assad's opponents would never accept any deal allowing him to remain in power.
But our correspondent says their disunity was openly on display, with constant disputes and walkouts.
Mr Annan's spokesman, Ahmed Fawzi, said he considered the Syrian acceptance of the peace plan an "important initial step" to help bring an end to the violence, but added that implementation was key.
Mr Annan thanked countries that have supported his attempts to mediate in the conflict. He is currently in Beijing for talks with Chinese Premier Wen Jiabao.
The six-point plan calls for Mr Assad's government to pull troops and heavy weapons out of population centres, and for all parties to allow a daily two-hour pause in the fighting in order for humanitarian aid to reach affected areas. The plan also requests that the authorities release those detained in the uprising.
However, it does not impose any deadline for Mr Assad, or call for him to leave power.
The BBC's Barbara Plett at the UN says despite the scepticism, this is a new situation because it is the first strategy for ending the conflict that has the backing of the entire Security Council, including Syria's allies Russia and China.
She says it seems it was this international unity which forced President Assad to accept the plan.
Mr Annan has written to Mr Assad urging him to put his commitments into immediate effect.
BBC Middle East editor Jeremy Bowen says the agreement is not as strongly worded as earlier UN resolutions, but it can be regarded as more pressure on Mr Assad and his government, which he seems to have decided that he cannot ignore.
However, implementing the plan is another matter, our correspondent adds.