Syrian troops have tortured children and used them as human shields on tanks to prevent attacks by opposition forces, according to a United Nations report.
The UN's Special Representative for Children and Armed Conflict said children were being tortured in detention and slaughtered in massacres. It said rebel forces were also using children on the front line.
UN special representative Radhika Coomaraswamy told the BBC her team had returned from Syria with "horrific" reports.
She said she had never seen a similar situation where children were not spared - and even targeted - in a conflict.
"Many former soldiers spoke about shooting into civilian areas, seeing children, young children being killed and maimed.
"We also had testimonies and saw children who had been tortured, and who carried the torture marks with them. We also heard of children being used - this was recounted to us by some children - of being put on tanks and being used as human shields so that the tanks would not be fired upon."
However, Ms Coomaraswamy also criticised the opposition Free Syrian Army for endangering children.
"For the first time we heard of children being recruited by the Free Syrian Army mainly in medical and service orientated jobs but still on the front line."
Ms Coomaraswamy said the suffering inflicted on children in Syria was unusual even for combat situations.
US fears new massacre in Syria
The US State Department says the government of Syria appears to be preparing a massacre in the opposition stronghold of Haffa.
The state department is citing reports from UN observers inside Syria and is warning the army there it will be held to account for killings.
"We are calling this out now in the hope that we can stop what could be a potential massacre," spokeswoman Victoria Nuland said.
Activists have accused Syrian government forces of being behind the killing of 108 people in the region of Houla and 78 others in the village of al-Qubeir this month.
Ms Nuland said Syrian President Bashar al-Assad's forces were using horrific tactics including firing on civilians from airborne helicopters and deploying plainclothes shabiha militia which the opposition has accused of slaughtering civilians in earlier attacks.
International mediator Kofi Annan, has described reports of mortar bombs, helicopters and tanks being used in Haffa, near the coast, as well as government shelling of the opposition stronghold of Homs.