Syrian authorities are systematically detaining and torturing children, the United Nations' human rights chief, Navi Pillay, has told the BBC.
|UN Human Rights Commissioner Navi Pillay: "President Assad could simply issue an order to stop the killings"|
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.
"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," she said.
"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."
On Tuesday, Radhika Coomaraswamy, the UN special representative for children and armed conflict, said they had received claims that the rebel Free Syrian Army was using children as fighters.
The UN says more than 9,000 people have been killed since the uprising against President Assad began a year ago.
Late on Tuesday, several Syrian dissident groups meeting in Istanbul agreed to recognise the Syrian National Council as the official representative of the Syrian people.
The BBC's Jonathan Head at the gathering said none of the delegates he spoke to believed President Assad was sincere, and the Syrian opposition 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.
Earlier, a spokesman for UN and Arab League envoy Kofi Annan said he considered the Syrian acceptance of his six-point peace plan an "important initial step" but that implementation was key.
|US Secretary of State Hillary Clinton said President Bashar al-Assad would be judged by events.|
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."
However, there were reports of further violence on Wednesday.
London-based opposition group the Syrian Observatory for Human Rights said that government forces backed by tanks had stormed the central town of Qalaat al-Madi at dawn, following a 17-day barrage.
A local activist told AFP news agency that fighters with the Free Syrian Army had withdrawn from the area.
Arab League foreign ministers meeting in Baghdad on Wednesday are expected to strongly back Mr Annan's peace mission, calling for talks between the Syrian opposition and government.
Syria is expected to dominate an Arab League summit in the Iraqi capital on Thursday.
Speaking to the BBC, Arab League Secretary General Nabil al-Arabi reiterated that the league had not called for the arming of the Syrian opposition. He said it was instead a decision for individual member states.
The six-point plan calls for Mr Assad's government to remove troops and heavy weapons from population centres and for all parties to allow a daily two-hour ceasefire for humanitarian aid to reach affected areas. The plan also requests that 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 that, despite the scepticism, this 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.