London: There is enough evidence to bring human rights charges against Syrian President Bashar al-Assad over his crackdown on protesters, UN rights Chief Navi Pillay has said.

In an interview Pillay said the president's role as commander of the security forces left him responsible for their actions during the unrest.

She also highlighted what she said was the regime's systematic targetting of children.

The Syrian army's use of heavy weapons against civilians in densely populated areas was a crime under international law, said Pillay.

"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," she said.

"President Assad could simply issue an order to stop the killings and the killings would stop...," the UN Human Rights Commissioner Pillay said.

"So this is the kind of thing that judges hearing cases on crimes against humanity will be looking at on command responsibility."

Pillay also spoke of evidence she had seen that the regime was systematically targeting children in its bid to stamp out resistance.

Hundreds of children had been detained and tortured, said the South African lawyer.

"It's just horrendous."

Pillay said the UN Security Council now had enough reliable evidence to warrant a referral to the International Criminal Court (ICC).

"There is no statute of limitations so people like him can go on for a very long time but one day they will have to face justice," she said, referring to Assad.

Syrian forces on Wednesday launched fresh attacks on rebel bastions as UN chief Ban Ki-moon urged Assad to immediately implement the UN-Arab League peace plan.